Sixth extinction, rivaling that of the dinosaurs, should join the big five, scientists say

ByEric Hand

Original link:  http://news.sciencemag.org/earth/2015/04/sixth-extinction-rivaling-dinosaurs-should-join-big-five-scientists-say?utm_source=facebook&utm_medium=social&utm_campaign=facebook

Earth has seen its share of catastrophes, the worst being the “big five” mass extinctions scientists traditionally talk about. Now, paleontologists are arguing that a sixth extinction, 260 million years ago, at the end of a geological age called the Capitanian, deserves to be a member of the exclusive club. In a new study, they offer evidence for a massive die-off in shallow, cool waters in what is now Norway. That finding, combined with previous evidence of extinctions in tropical waters, means that the Capitanian was a global catastrophe.

“It’s the first time we can say this is a true global extinction,” says David Bond, a paleontologist at the University of Hull in the United Kingdom. Bond led a study that was published online this week in the Geological Society of America Bulletin. He adds that in magnitude, the Capitanian event was on par with the dinosaur-killing extinction 66 million year ago. “I’d put this up there with it, albeit with slightly less attractive victims,” Bond says.

Interest in the Capitanian began in the early 1990s, when paleontologists found evidence for fossil extinctions in rock formations in China. The rocks had originally formed on the floor of a shallow tropical sea. Most foraminifera—tiny, shelled protozoans—were wiped out, along with many species of clamlike brachiopods. There was also a possible trigger to blame: a set of ancient volcanic outbursts in China that solidified into rocks called the Emeishan Traps. The hot flood basalts would have released huge amounts of sulfur and carbon dioxide, potentially causing a quick global chill followed by a longer period of global warming. The gases could have also driven acidification and oxygen depletion in the oceans. Many scientists think that a similar massive burst of volcanic activity in Siberia touched off the biggest extinction of all time, just 8 million years later, at the end of the Permian period.
But the older, less studied Capitanian extinction has been dogged by criticism that it may have been a regional event, or just part of a gradual trend en route to the larger Permian extinction. Some of those criticisms may be quelled by the new evidence, which comes from Spitsbergen, the largest island in the Svalbard archipelago off the coast of Norway in the Arctic Ocean. There, Bond and his colleagues examined chert rocks—silica formations, created by the skeletons of dead sponges, that also contain many species of brachiopods. At the time, the rocks would have been forming in tens of meters of cooler water at midlatitudes. But at a stark point in the rock record, the fossils disappeared.

“They all drop out,” says study co-author Paul Wignall, a paleontologist at the University of Leeds in the United Kingdom. “It’s like a blackout zone and there’s nothing around.” A little further in the rock record, a few brachiopod species recover, Wignall says, and then mollusks take over en masse, before the devastation of the Permian extinction, 8 million years later.

The research team had a hard time tying the new record to the same moment in fossil records in China. Isotopic dating systems are too uncertain to provide a helpful absolute date. Another standard biostratigraphic method—linking the timing of different rock layers by the comings and goings of fossilized teeth of tiny eellike creatures called conodonts—also couldn’t be used, because the same species didn’t live in cool and tropical waters. Instead, the team points out that similar swings in different isotopes’ levels, occurring in both parts of the world, suggest that the two regions were experiencing the same changes in ocean chemistry at the same time.

That’s part of the problem, says Matthew Clapham, a paleontologist at the University of California, Santa Cruz. He thinks the study team has dated something a bit younger—maybe 255 million years old. “They’ve definitely identified a real event, which is really interesting,” he says. “Their age model is less convincing.” He also says that recent work in China on the extent of the Capitanian extinction across different species shows it may not have been quite as bad as originally thought. Clapham thinks the Capitanian is probably 30th or 40th in the hierarchy of extinctions, not sixth.

But Bond is still convinced that the Capitanian will go down in the history books as one of the world’s worst. “You have to change a lot of people’s minds,” he says. He is now studying fossil records in Russia and Greenland that could further buttress his arguments for a global disaster. Clapham, too, wants to see more work done on this enigmatic stretch of Earth history. “It’s a very mysterious event—it’s an interesting thing to study,” he says.

 

Posted in Earth, Paleontology

Science| DOI: 10.1126/science.aab2504

Potential applications of graphene

From Wikipedia, the free encyclopedia

Potential graphene applications include lightweight, thin, flexible, yet durable display screens, electric circuits, and solar cells, as well as various medical, chemical and industrial processes enhanced or enabled by the use of new graphene materials.^[1]

In 2008, graphene produced by exfoliation was one of the most expensive materials on Earth, with a sample the area of a cross section of a human hair costing more than $1,000 as of April 2008 (about $100,000,000/cm²).^[2] Since then, exfoliation procedures have been scaled up, and now companies sell graphene in large quantities.^[3] The price of epitaxial graphene on Silicon carbide is dominated by the substrate price, which was approximately $100/cm² as of 2009.

Hong and his team in South Korea pioneered the synthesis of large-scale graphene films using chemical vapour deposition (CVD) on thin nickel layers, which triggered research on practical applications,^[4] with wafer sizes up to 30 inches (760 mm) reported.^[5]

In 2013, the European Union made a €1 billion grant to be used for research into potential graphene applications.^[6]

In 2013 the Graphene Flagship consortium formed, including Chalmers University of Technology and seven other European universities and research centers, along with Nokia.^[7]

Medicine

Tissue engineering

Graphene has been investigated for tissue engineering. It has been used as a reinforcing agent to improve the mechanical properties of biodegradable polymeric nanocomposites for engineering bone tissue applications.^[8] Dispersion of low weight % of graphene (~0.02 wt.%) increased in compressive and flexural mechanical properties of polymeric nanocomposites.

Contrast agents/bioimaging

Functionalized and surfactant dispersed graphene solutions have been designed as blood pool MRI contrast agents.^[9] Additionally, iodine and manganese incorporating graphene nanoparticles have served as multimodal MRI-CT contrast agents.^[10] Graphene micro- and nano-particles have served as contrast agents for photoacoustic and thermoacoustic tomography.^[11] Graphene has also been reported to be efficiently taken up cancerous cells thereby enabling the design of drug delivery agents for cancer therapy.^[12] Graphene nanoparticles of various morphologies are non-toxic at low concentrations and do not alter stem cell differentiation suggesting that they may be safe to use for biomedical applications.^[13]

Polymerase chain reaction

Graphene is reported to have enhanced PCR by increasing the yield of DNA product.^[14] Experiments revealed that graphene's thermal conductivity could be the main factor behind this result. Graphene yields DNA product equivalent to positive control with up to 65% reduction in PCR cycles.^{[citation needed]}

Devices

Graphene's modifiable chemistry, large surface area, atomic thickness and molecularly gatable structure make antibody-functionalized graphene sheets excellent candidates for mammalian and microbial detection and diagnosis devices.^[15] Graphene is so thin water has near-perfect wetting transparency which is an important property particularly in developing bio-sensor applications.^[16] This means that a sensors coated in graphene have as much contact with an aqueous system as an uncoated sensor, while it remains protected mechanically from its environment.

Energy of the electrons with wavenumber k in graphene, calculated in the Tight Binding-approximation. The unoccupied (occupied) states, colored in blue–red (yellow–green), touch each other without energy gap exactly at the above-mentioned six k-vectors.

Integration of graphene (thickness of 6990340000000000000♠0.34 nm) layers as nanoelectrodes into a nanopore^[17] can potentially solve a bottleneck for nanopore-based single-molecule DNA sequencing.

On November 20, 2013 the Bill & Melinda Gates Foundation awarded $100,000 'to develop new elastic composite materials for condoms containing nanomaterials like graphene'.^[18]

In 2014, graphene-based, transparent (across infrared to ultraviolet frequencies), flexible, implantable medical sensor microarrays were announced that allow the viewing of brain tissue hidden by implants. Optical transparency was >90%. Applications demonstrated include optogenetic activation of focal cortical areas, in vivo imaging of cortical vasculature via fluorescence microscopy and 3D optical coherence tomography.^[19][20]

Drug delivery[edit]

• Researchers in Monash University discovered that the sheet of graphene oxide can be transformed into liquid crystal droplets spontaneously – like a polymer - simply by placing the material in a solution and manipulating the pH. The graphene droplets change their structure at the presence of an external magnetic field. This finding opens the door for potential use of carrying drug in the graphene droplets and drug release upon reaching the targeted tissue when the droplets change shape under the magnetic field. Another possible application is in disease detection if graphene is found to change shape at the presence of certain disease markers such as toxins.^[21][22]

• A graphene ‘flying carpet’ was demonstrated to deliver two anti-cancer drugs sequentially to the lung tumor cells (A549 cell) in a mouse model. Doxorubicin (DOX) is embedded onto the graphene sheet, while the molecules of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) are linked to the nanostructure via short peptide chains. Injected intravenously, the graphene strips with the drug playload preferentially concentrate to the cancer cells due to common blood vessel leakage around the tumor. Receptors on the cancer cell membrane bind TRAIL and cell surface enzymes clip the peptide thus release the drug onto the cell surface. Without the bulky TRAIL, the graphene strips with the embedded DOX are swallowed into the cells. The intracellular acidic environment promotes DOX’s release from graphene. TRAIL on the cell surface triggers the apoptosis while DOX attacks the nucleus. These two drugs work synergistically and were found to be more effective than either drug alone.^[23][24]

Biomicrorobotics

Researchers demonstrated a nanoscale biomicrorobot (or cytobot) made by cladding a living endospore cell with graphene quantum dots. The device acted as a humidity sensor.^[25]

Testing

In 2014 a graphene based blood glucose testing product was announced.^[26][27]

Electronics

For integrated circuits, graphene has a high carrier mobility, as well as low noise, allowing it to be used as the channel in a field-effect transistor. Single sheets of graphene are hard to produce and even harder to make on an appropriate substrate.^[28]

In 2008, the smallest transistor so far, one atom thick, 10 atoms wide was made of graphene.^[29] IBM announced in December 2008 that they had fabricated and characterized graphene transistors operating at GHz frequencies.^[30] In May 2009, an n-type transistor was announced meaning that both n and p-type graphene transistors had been created.^[31][32] A functional graphene integrated circuit was demonstrated – a complementary inverter consisting of one p- and one n-type graphene transistor.^[33] However, this inverter suffered from a very low voltage gain.

According to a January 2010 report,^[34] graphene was epitaxially grown on SiC in a quantity and with quality suitable for mass production of integrated circuits. At high temperatures, the quantum Hall effect could be measured in these samples. IBM built 'processors' using 100 GHz transistors on 2-inch (51 mm) graphene sheets.^[35]

In June 2011, IBM researchers announced that they had succeeded in creating the first graphene-based integrated circuit, a broadband radio mixer.^[36] The circuit handled frequencies up to 10 GHz. Its performance was unaffected by temperatures up to 127 °C.

In June 2013 an 8 transistor 1.28 GHz ring oscillator circuit was described.^[37]

Transistors

Graphene exhibits a pronounced response to perpendicular external electric fields, potentially forming field-effect transistors (FET). A 2004 paper documented FETs with an on-off ratio of ~30 at room temperature.^{[citation needed]} A 2006 paper announced an all-graphene planar FET with side gates.^[38] Their devices showed changes of 2% at cryogenic temperatures. The first top-gated FET (on–off ratio of <2 2007.="" class="reference" demonstrated="" id="cite_ref-39" in="" sup="" was="">[39]

Graphene nanoribbons may prove generally capable of replacing silicon as a semiconductor.^[40]
US patent 7015142  for graphene-based electronics was issued in 2006. In 2008, researchers at MIT Lincoln Lab produced hundreds of transistors on a single chip^[41] and in 2009, very high frequency transistors were produced at Hughes Research Laboratories.^[42]

A 2008 paper demonstrated a switching effect based on a reversible chemical modification of the graphene layer that gives an on–off ratio of greater than six orders of magnitude. These reversible switches could potentially be employed in nonvolatile memories.^[43]

In 2009, researchers demonstrated four different types of logic gates, each composed of a single graphene transistor.^[44]

Practical uses for these circuits are limited by the very small voltage gain they exhibit. Typically, the amplitude of the output signal is about 40 times less than that of the input signal. Moreover, none of these circuits operated at frequencies higher than 25 kHz.

In the same year, tight-binding numerical simulations^[45] demonstrated that the band-gap induced in graphene bilayer field effect transistors is not sufficiently large for high-performance transistors for digital applications, but can be sufficient for ultra-low voltage applications, when exploiting a tunnel-FET architecture.^[46]

In February 2010, researchers announced transistors with an on/off rate of 100 gigahertz, far exceeding the rates of previous attempts, and exceeding the speed of silicon transistors with an equal gate length. The 6993239990000099999♠240 nm devices were made with conventional silicon-manufacturing equipment.^[47][48][49]

In November 2011, researchers used 3d printing (additive manufacturing) as a method for fabricating graphene devices.^[50]

In 2013, researchers demonstrated graphene's high mobility in a detector that allows broad band frequency selectivity ranging from the THz to IR region (0.76–33 THz)^[51] A separate group created a terahertz-speed transistor with bistable characteristics, which means that the device can spontaneously switch between two electronic states. The device consists of two layers of graphene separated by an insulating layer of boron nitride a few atomic layers thick. Electrons move through this barrier by quantum tunneling. These new transistors exhibit “negative differential conductance,” whereby the same electrical current flows at two different applied voltages.^[52]

Graphene does not have an energy band-gap, which presents a hurdle for its applications in digital logic gates. The efforts to induce a band-gap in graphene via quantum confinement or surface functionalization have not resulted in a breakthrough. The negative differential resistance experimentally observed in graphene field-effect transistors of "conventional" design allows for construction of viable non-Boolean computational architectures with the gap-less graphene. The negative differential resistance — observed under certain biasing schemes — is an intrinsic property of graphene resulting from its symmetric band structure. The results present a conceptual change in graphene research and indicate an alternative route for graphene's applications in information processing.^[53]

In 2013 researchers reported the creation of transistors printed on flexible plastic that operate at 25 gigahertz, sufficient for communications circuits and that can be fabricated at scale. The researchers first fabricate the non-graphene-containing structures—the electrodes and gates—on plastic sheets. Separately, they grow large graphene sheets on metal, then peel it off and transfer it to the plastic. Finally, they top the sheet with a waterproof layer. The devices work after being soaked in water, and are flexible enough to be folded.^[54]

Trilayer graphene

An electric field can change trilayer graphene's crystal structure, transforming its behavior from metal-like to semiconductor-like. A sharp metal scanning tunneling microscopy tip was able to move the domain border between the upper and lower graphene configurations. One side of the material behaves as a metal, while the other side behaves as a semiconductor. Trilayer graphene can be stacked in either Bernal or rhombohedral configurations, which can exist in a single flake. The two domains are separated by a precise boundary at which the middle layer is strained to accommodate the transition from one stacking pattern to the other.^[55]

Silicon transistors function as either p-type or n-type semiconductors, whereas graphene could operate as both. This lowers costs and is more versatile. The technique provides the basis for a field-effect transistor. Scalable manufacturing techiques have yet to be developed.^[55]

In trilayer graphene, the two stacking configurations exhibit very different electronic properties. The region between them consists of a localized strain soliton where the carbon atoms of one graphene layer shift by the carbon–carbon bond distance. The free-energy difference between the two stacking configurations scales quadratically with electric field, favoring rhombohedral stacking as the electric field increases.^[55]

This ability to control the stacking order opens the way to new devices that combine structural and electrical properties.^[55][56]

Graphene-based transistors could be much thinner than modern silicon devices, allowing faster and smaller configurations.^{[citation needed]}

Transparent conducting electrodes

Graphene's high electrical conductivity and high optical transparency make it a candidate for transparent conducting electrodes, required for such applications as touchscreens, liquid crystal displays, organic photovoltaic cells, and organic light-emitting diodes. In particular, graphene's mechanical strength and flexibility are advantageous compared to indium tin oxide, which is brittle. Graphene films may be deposited from solution over large areas.^[57][58]

Large-area, continuous, transparent and highly conducting few-layered graphene films were produced by chemical vapor deposition and used as anodes for application in photovoltaic devices. A power conversion efficiency (PCE) up to 1.71% was demonstrated, which is 55.2% of the PCE of a control device based on indium tin oxide.^[59]

Organic light-emitting diodes (OLEDs) with graphene anodes have been demonstrated.^[60] The electronic and optical performance of graphene-based devices are similar to devices made with indium tin oxide.

A carbon-based device called a light-emitting electrochemical cell (LEC) was demonstrated with chemically-derived graphene as the cathode and the conductive polymer PEDOT as the anode.^[61] Unlike its predecessors, this device contains only carbon-based electrodes, with no metal.^{[citation needed]}

In 2014 a prototype graphene-based flexible display was demonstrated.^[62]

Frequency multiplier

In 2009, researchers built experimental graphene frequency multipliers that take an incoming signal of a certain frequency and output a signal at a multiple of that frequency.^[63]

Optoelectronics

Graphene strongly interacts with photons, with the potential for direct band-gap creation. This is promising for optoelectronic and nanophotonic devices. Light interaction arises due to the Van Hove singularity. Graphene displays different time scales in response to photon interaction, ranging from femtoseconds (ultra-fast) to picoseconds. Potential uses include transparent films, touch screens and light emitters or as a plasmonic device that confines light and alters wavelengths.^[64]

Hall effect sensors

Due to extremely high electron mobility, graphene may be used for production of highly sensitive Hall effect sensors.^[65] Potential application of such sensors is connected with DC current transformers for special applications.^{[citation needed]}

Quantum dots

Graphene quantum dots (GQDs) keep all dimensions less than 10 nm. Their size and edge crystallography govern their electrical, magnetic, optical and chemical properties. GQDs can be produced via graphite nanotomy^[66] or via bottom-up, solution-based routes (Diels-Alder, cyclotrimerization and/or cyclodehydrogenation reactions).^[67]
GQDs with controlled structure can be incorporated into applications in electronics, optoelectronics and electromagnetics. Quantum confinement can be created by changing the width of graphene nanoribbons (GNRs) at selected points along the ribbon.^[29][68] It is studied as a catalyst for fuel cells.^[69]

Organic electronics

A semiconducting polymer (poly(3-hexylthiophene)^[70] placed on top of single-layer graphene vertically conducts electric charge better than on a thin layer of silicon. A 50 nm thick polymer film conducted charge about 50 times better than a 10 nm thick film, potentially because the former consists of a mosaic of variably-oriented crystallites forms a continuous pathway of interconnected crystals. In a thin film or on silicon,^[70] plate-like crystallites are oriented parallel to the graphene layer. Uses include solar cells.^[71]

Light processing

Optical modulator

When the Fermi level of graphene is tuned, its optical absorption can be changed. In 2011, researchers reported the first graphene-based optical modulator. Operating at 7009120000000000000♠1.2 GHz without a temperature controller, this modulator has a broad bandwidth (from 1.3 to 1.6 μm) and small footprint (~7001250000000000000♠25 μm²).^[72]

Infrared light detection

Graphene, reacts to the infrared spectrum at room temperature, albeit with sensitivity 100 to 1000 times too low for practical applications. However, two graphene layers separated by an insulator allowed an electric field produced by holes left by photo-freed electrons in one layer to affect a current running through the other layer. The process produces little heat, making it suitable for use in night-vision optics. The sandwich is thin enough to be integrated in handheld devices, eyeglass-mounted computers and even contact lenses.^[73]

Energy

Generation

Ethanol distillation

Graphene oxide membranes allow water vapor to pass through, but are impermeable to other liquids and gases.^[74] This phenomenon has been used for further distilling of vodka to higher alcohol concentrations, in a room-temperature laboratory, without the application of heat or vacuum as used in traditional distillation methods.^[75] Further development and commercialization of such membranes could revolutionize the economics of biofuel production and the alcoholic beverage industry.^{[citation needed]}

Solar cells

Graphene has a unique combination of high electrical conductivity and optical transparency, which make it a candidate for use in solar cells. A single sheet of graphene is a zero-bandgap semiconductor whose charge carriers are delocalized over large areas, implying that carrier scattering does not occur. Because this material only absorbs 2.6% of green light and 2.3% of red light,^[76] it is a candidate for applications requiring a transparent conductor.

Graphene can be assembled into a film electrode with low roughness. However, graphene films produced via solution processing contain lattice defects and grain boundaries that act as recombination centers and decrease the material's electrical conductivity. Thus, these films must be made thicker than one atomic layer to obtain useful sheet resistances. This added resistance can be combatted by incorporating conductive filler materials, such as a silica matrix. Reduced graphene film's electrical conductivity can be improved by attaching large aromatic molecules such as pyrene-1-sulfonic acid sodium salt (PyS) and the disodium salt of 3,4,9,10-perylenetetracarboxylic diimide bisbenzenesulfonic acid (PDI). These molecules, under high temperatures, facilitate better π-conjugation of the graphene basal plane. Graphene films have high transparency in the visible and near-infrared regions and are chemically and thermally stable.^[77]

For graphene to be used in commercial solar cells, large-scale production is required. However, no scalable process for producing graphene is available, including the peeling of pyrolytic graphene or thermal decomposition of silicon carbide.^[77]

Graphene's high charge mobilities recommend it for use as a charge collector and transporter in photovoltaics (PV). Using graphene as a photoactive material requires its bandgap to be 1.4–1.9 eV. In 2010, single cell efficiencies of nanostructured graphene-based PVs of over 12% were achieved. According to P. Mukhopadhyay and R. K. Gupta organic photovoltaics could be "devices in which semiconducting graphene is used as the photoactive material and metallic graphene is used as the conductive electrodes".^[77]

In 2010, Xinming Li and Hongwei Zhu from Tsinghua University first reported graphene-silicon heterojunction solar cell, where graphene served as a transparent electrode and introduced a built-in electric field near the interface between the graphene and n-type silicon to help collect photo-generated carriers. More studies promote this new type of photovoltaic device.^[78] For example, in 2012 researchers from the University of Florida reported efficiency of 8.6% for a prototype cell consisting of a wafer of silicon coated with a layer of graphene doped with trifluoromethanesulfonyl-amide (TFSA). Furthermore, Xinming Li found chemical doping could improve the graphene characteristics and significantly enhance the efficiency of graphene-silicon solar cell to 9.6% in 2013.^[79]
In 2015 researchers reported efficiency of 15.6% by choosing the optimal oxide thickness on the silicon.^[80]

In 2013 another team claimed to have reached 15.6% percent using a combination of titanium oxide and graphene as a charge collector and perovskite as a sunlight absorber. The device is manufacturable at temperatures under 150 °C (302 °F) using solution-based deposition. This lowers production costs and offers the potential using flexible plastics.^[81]

Large scale production of highly transparent graphene films by chemical vapor deposition was achieved in 2008. In this process, ultra-thin graphene sheets are created by first depositing carbon atoms in the form of graphene films on a nickel plate from methane gas. A protective layer of thermoplastic is laid over the graphene layer and the nickel underneath is dissolved in an acid bath. The final step is to attach the plastic-protected graphene to a flexible polymer sheet, which can then be incorporated into an OPV cell. Graphene/polymer sheets range in size up to 150 square centimeters and can be used to create dense arrays of flexible OPV cells. It may eventually be possible to run printing presses covering extensive areas with inexpensive solar cells, much like newspaper presses print newspapers (roll-to-roll).^[82]

Silicon generates only one current-driving electron for each photon it absorbs, while graphene can produce multiple electrons. Solar cells made with graphene could offer 60% conversion efficiency – double the widely accepted maximum efficiency of silicon cells.^[83]

Fuel cells

Appropriately perforated graphene (and hexagonal boron nitride hBN) can allow protons to pass through it, offering the potential for using graphene monolayers as a barrier that blocks hydrogen atoms but not protons/ionized hydrogen (hydrogen atoms with their electrons stripped off). They could even be used to extract hydrogen gas out of the atmosphere that could power electric generators with ambient air.^[84]

The membranes are more effective at elevated temperatures and when covered with catalytic nanoparticles such as platinum.^[84]

Graphene could solve a major problem for fuel cells: fuel crossover that reduces efficiency and durability.^[84]

At room temperature, proton conductivity with monolayer hBN, outperforms graphene, with resistivity to proton flow of about 10 Ω cm² and a low activation energy of about 0.3 electronvolts. At higher temperatures, graphene outperforms with resistivity estimated to fall below 10⁻³ Ω cm² above 250 degrees Celsius.^[85]

In another project, protons easily pass through slightly imperfect graphene membranes on fused silica in water.^[86] The membrane was exposed to cycles of high and low pH. Protons transferred reversibly from the aqueous phase through the graphene to the other side where they undergo acid–base chemistry with silica hydroxyl groups. Computer simulations indicated energy barriers of 0.61–0.75 eV for hydroxyl-terminated atomic defects that participate in a Grotthuss-type relay, while pyrylium-like ether terminations did not.^[87]

Storage

Supercapacitor

Due to graphene's high surface area to mass ratio, one potential application is in the conductive plates of supercapacitors.^[88]

In February 2013 researchers announced a novel technique to produce graphene supercapacitors based on the DVD burner reduction approach.^[89]

In 2014 a supercapacitor was announced that was claimed to achieve energy density comparable to current lithium-ion batteries.^[26][27]

In 2015 the technique was adapted to produce stacked, 3-D supercapacitors. Laser-induced graphene was produced on both sides of a polymer sheet. The sections were then stacked, separated by solid electrolytes, making multiple microsupercapacitors. The stacked configuration substantially increased the energy density of the result. In testing, the researchers charged and discharged the devices for thousands of cycles with almost no loss of capacitance.^[90]
The resulting devices were mechanically flexible, surviving 8,000 bending cycles. This makes them potentially suitable for rolling in a cylindrical configuration.  Solid-state polymeric electrolyte-based devices exhibit areal capacitance of >9 mF/cm2 at a current density of 0.02 mA/cm2, over twice that of conventional aqueous electrolytes.^[91]

Also in 2015 another project announced a microsupercapacitor that is small enough to fit in wearable or implantable devices. Just one-fifth the thickness of a sheet of paper, it is capable of holding more than twice as much charge as a comparable thin-film lithium battery. The design employed laser-scribed graphene, or LSG with manganese dioxide. They can be fabricated without extreme temperatures or expensive “dry rooms”. Their capacity is six times that of commercially available supercapacitors.^[92] The device reached volumetric capacitance of over 1,100 F/cm3. This corresponds to a specific capacitance of the constituent MnO2 of 1,145 F/g, close to the theoretical maximum of 1,380 F/g. Energy density varies between 22 and 42 Wh/l depending on device configuration.^[93]

Electrode for Li-ion batteries

Stable Li-ion cycling has recently been demonstrated in bi- and few layer graphene films grown on nickel substrates,^[94] while single layer graphene films have been demonstrated as a protective layer against corrosion in battery components such as the battery case.^[95] This creates possibilities for flexible electrodes for microscale Li-ion batteries where the anode acts as the active material as well as the current collector.^[96]
There are also silicon-graphene anode Li-ion batteries.^[97]

Hydrogen storage

Hydrogenation-assisted graphene origami (HAGO) was used to cause approximately square graphene sheets to fold into a cage can store hydrogen at 9.5 percent by weight. The U.S. Department of Energy had set a goal of 7.5 percent hydrogen by 2020. An electric field causes the box to open and close.^[98]

Rechargeable battery

Researchers at Northwestern University built a lithium-ion battery made of graphene and silicon, which was claimed to last over a week on a single charge and only took 15 minutes to charge.^[99]

Sensors

Molecular adsorbtion

Theoretically graphene makes an excellent sensor due to its 2D structure. The fact that its entire volume is exposed to its surrounding environment makes it very efficient to detect adsorbed molecules. However, similar to carbon nanotubes, graphene has no dangling bonds on its surface. Gaseous molecules cannot be readily adsorbed onto graphene surfaces, so intrinsically graphene is insensitive.^[100] The sensitivity of graphene chemical gas sensors can be dramatically enhanced by functionalization, for example, coating the film with a thin layer of certain polymers. The thin polymer layer acts like a concentrator that absorbs gaseous molecules. The molecule absorption introduces a local change in electrical resistance of graphene sensors. While this effect occurs in other materials, graphene is superior due to its high electrical conductivity (even when few carriers are present) and low noise, which makes this change in resistance detectable.^[101]

Piezoelectric effect

Density functional theory simulations predict that depositing certain adatoms on graphene can render it piezoelectrically responsive to an electric field applied in the out-of-plane direction. This type of locally engineered piezoelectricity is similar in magnitude to that of bulk piezoelectric materials and makes graphene a candidate for control and sensing in nanoscale devices.^[102]

Body motion

Rubber bands infused with graphene("G-bands") can be used as inexpensive body sensors. The bands remain pliable and can be used as a sensor to measure breathing, heart rate, or movement. Lightweight sensor suits for vulnerable patients could make it possible to remotely monitor subtle movement. These sensors display 7001100000000000000♠10-fold increases in resistance and work at strains exceeding 800%. Gauge factors of up to 35 were observed. Such sensors can function at vibration frequencies of at least 160 Hz. At 60 Hz, strains of at least 6% at strain rates exceeding 6000%/s can be monitored.^[103]

Environmental

Contaminant removal

Graphene oxide is non-toxic and biodegradable. Its surface is covered with epoxy, hydroxyl, and carboxyl groups that interact with cations and anions. It is soluble in water and forms stable colloid suspensions in other liquids because it is amphiphilic (able to mix with water or oil). Dispersed in liquids it shows excellent sorption capacities. It can remove copper, cobalt, cadmium, arsenate and organic solvents.^[104]

In 2013 it was shown to be able to remove radioactive nuclides from water, including radioactive isotopes of actinides (elements with atomic numbers 89 to 103, including thorium, uranium, neptunium, plutonium and americium) and lanthanides (the ‘rare earths’ with atomic numbers 57 to 71, including europium).^[104]

Even at concentrations < 0.1 g/L, radionuclide sorption proceeds rapidly. At pH between 4 and 8, graphene oxide removes over 90% of nuclides, including uranium and europium.  At pH >7, more than 70% of strontium and technicium are removed with up to 20% of neptunium.^[104]

Water filtration

Research suggests that graphene filters could outperform other techniques of desalination by a significant margin.^[105]

Other

Plasmonics and metamaterials

Graphene accommodates a plasmonic surface mode, observed recently via near field infrared optical microscopy techniques.^[106][107] and infrared spectroscopy ^[108] Potential applications are in the terahertz to midinfrared frequencies,^[109] such as terahertz and midinfrared light modulators, passive terahertz filters, midinfrared photodetectors and biosensors.

Lubricant

Scientists discovered using graphene as a lubricant works better than traditionally used graphite. A one atom thick layer of graphene in between a steel ball and steel disc lasted for 6,500 cycles. Conventional lubricants lasted 1,000 cycles.^[110]

Radio wave absorption

Stacked graphene layers on a quartz substrate increased the absorption of millimeter (radio) waves by 90 per cent over 125 – 165 GHz bandwidth, extensible to microwave and low-terahertz frequencies, while remaining transparent to visible light. For example, graphene could be used as a coating for buildings or windows to block radio waves. Absorption is a result of mutually coupled Fabry–Perot resonators represented by each graphene-quartz substrate. A repeated transfer-and-etch process was used to control surface resistivity.^[111][112]

Redox

Graphene oxide can be reversibly reduced and oxidized using electrical stimulus. Controlled reduction and oxidation in two-terminal devices containing multilayer graphene oxide films are shown to result in switching between partially reduced graphene oxide and graphene, a process that modifies electronic and optical properties. Oxidation and reduction are related to resistive switching.^[113]

Nanoantennas

A graphene-based plasmonic nano-antenna (GPN) can operate efficiently at millimeter radio wavelengths. The wavelength of surface plasmon polaritons for a given frequency is several hundred times smaller than the wavelength of freely propagating electromagnetic waves of the same frequency. These speed and size differences enable efficient graphene-based antennas to be far smaller than conventional alternatives. The latter operate at frequencies 100-1000 times larger than GPNs, producing .01-.001 as many photons.^[114]

An electromagnetic (EM) wave directed vertically onto a graphene surface excites the graphene into oscillations that interact with those in the dielectric on which the graphene is mounted, thereby forming surface plasmon polaritons (SPP). When the antenna becomes resonant (an integral number of SPP wavelengths fit into the physical dimensions of the graphene), the SPP/EM coupling increases greatly, efficiently transferring energy between the two.^[114]

A phased array antenna 100 µm in diameter could produce 300 GHz beams only a few degrees in diameter, instead of the 180 degree radiation from tsa conventional metal antenna of that size. Potential uses include smart dust, low-power terabit wireless networks^[114] and photonics.^[115]

A nanoscale gold rod antenna captured and transformed EM energy into graphene plasmons, analogous to a radio antenna converting radio waves into electromagnetic waves in a metal cable. The plasmon wavefronts can be directly controlled by adjusting antenna geometry. The waves were focused (by curving the antenna) and refracted (by a prism-shaped graphene bilayer because the conductivity in the two-atom-thick prism is larger than in the surrounding one-atom-thick layer.)^[115]

Sound transducers

Graphene provides relatively good frequency response, suggesting uses in audio speakers. Its light weight may make it suitable for microphones as well.^[116]

Waterproof coating

Graphene could potentially usher in a new generation of waterproof devices whose chassis may not need to be sealed like today's devices.^{[99][dubious – discuss]}

Coolant additive

Graphene's high thermal conductivity suggests that it could be used as an additive in coolants. Preliminary research work showed that 5% graphene by volume can enhance the thermal conductivity of a base fluid by 86%.^[117]
Another application due to graphene's enhanced thermal conductivity was found in PCR.^[14]

Reference material

Graphene's properties suggest it as a reference material for characterizing electroconductive and transparent materials. One layer of graphene absorbs 2.3% of red light.^[118]

This property was used to define the conductivity of transparency that combines sheet resistance and transparency.
This parameter was used to compare materials without the use of two independent parameters.^[119]

Thermal management

In 2011, researchers reported that a three-dimensional, vertically aligned, functionalized multilayer graphene architecture can be an approach for graphene-based thermal interfacial materials (TIMs) with superior thermal conductivity and ultra-low interfacial thermal resistance between graphene and metal.^[120]

Graphene-metal composites can be utilized in thermal interface materials.^[121]

Adding a layer of graphene to each side of a copper film increased the metal's heat-conducting properties up to 24%. This suggests the possibility of using them for semiconductor interconnects in computer chips. The improvement is the result of changes in copper’s nano- and microstructure, not from graphene’s independent action as an additional heat conducting channel. High temperature chemical vapor deposition stimulates grain size growth in copper films. The larger grain sizes improve heat conduction. The heat conduction improvement was more pronounced in thinner copper films, which is useful as copper interconnects shrink.^[122]

Structural material

Graphene's strength, stiffness and lightness suggested it for use with carbon fiber. Graphene has been used as a reinforcing agent to improve the mechanical properties of biodegradable polymeric nanocomposites for engineering bone tissue.^[123]

Catalyst

In 2014, researchers at The University of Western Australia discovered nano sized fragments of graphene can speed up the rate of chemical reactions.^[124]

Graphene device makes ultrafast light to energy conversion possible

By Colin Jeffrey

Original link:  http://www.gizmag.com/graphene-ultrafast-light-energy-conversion-photodetector-semiconductor/37005/

Using layers of graphene, scientists claim to have created a
photodetector that converts light to energy in less than 50 quadrillionths
of a second (Image: ICFO/Achim Woessner)

Converting light to electricity is one of the pillars of modern electronics, with the process essential for the operation of everything from solar cells and TV remote control receivers through to laser communications and astronomical telescopes. These devices rely on the swift and effective operation of this technology, especially in scientific equipment, to ensure the most efficient conversion rates possible. In this vein, researchers from the Institute of Photonic Sciences (Institut de Ciències Fotòniques/ICFO) in Barcelona have demonstrated a graphene-based photodetector they claim converts light into electricity in less than 50 quadrillionths of a second.

Graphene has already been identified as a superior substance for the transformation of photons to electrical current, even in the infrared part of the spectrum. However, prior to the ICFO research, it was unclear exactly how fast graphene would react when subjected to ultra-rapid bursts of light energy.

To test the speed of conversion, the ICFO team – in collaboration with scientists from MIT and the University of California, Riverside – utilized an arrangement consisting of graphene film layers set up as a p-n (positive-negative) junction semiconductor, a sub-50 femtosecond, titanium-sapphire, pulse-shaped laser to provide the ultrafast flashes of light, along with an ultra-sensitive pulse detector to capture the speed of conversion to electrical energy.

When this arrangement was fired up and tested, the scientists realized that the photovoltage generation time was occurring at a rate of better than 50 femtoseconds (or 50 quadrillionths of a second).

According to the researchers, this blistering speed of conversion is due to the structure of graphene which allows the exceptionally rapid and effective interaction between all of the conduction band carriers contained within it. In other words, the excitation of the molecules of graphene by the laser pulses causes the electrons in the material to heat up, and stay hot, while the carbon lattice underlying the structure remains cool. And, as the electrons in the laser-excited graphene do not cool down rapidly because they do not easily recouple with the graphene lattice, they remain in that state and transfer their energy much more rapidly.

As such, constant laser-pulse excitation of an area of graphene quickly results in superfast electron distribution within the material at constantly elevated electron temperatures. This rapid conversion to electron heat is then converted into a voltage at the p-n junction of two graphene regions.

Significantly, this "hot-carrier" generation is quite different from the operation of standard semiconductor devices. This is because their operation is dependent upon overcoming of the binding electron energy inherent in the material for an incoming photon to dislodge an electron and create an electrical current. In the ICFO device, the continued excitation of electrons above this band-gap level results in the much faster and easier movement of them when subjected to incoming photons to create an electric current.

Though it is early days in the study of such devices, the practical upshot of this research may be in the eventual production of novel types of ultrafast and extremely effective photodetectors and energy-harvesting devices. And, given that the basic operating principles of hot-carrier graphene devices are substantially different from traditional silicon or germanium semiconductors, an entirely new stream of electronic components that take advantage of this phenomenon may evolve.

The findings of this work have recently been published in the journal Nature Nanotechnology.

Source: ICFO

Raman spectroscopy

From Wikipedia, the free encyclopedia

Energy-level diagram showing the states involved in Raman signal. The line thickness is roughly proportional to the signal strength from the different transitions.

Raman spectroscopy (/ˈrɑːmən/; named after Sir C. V. Raman) is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a system.^[1] Raman spectroscopy is commonly used in chemistry to provide a fingerprint by which molecules can be identified.

It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the system. Infrared spectroscopy yields similar, but complementary, information.

Typically, a sample is illuminated with a laser beam. Electromagnetic radiation from the illuminated spot is collected with a lens and sent through a monochromator. Elastic scattered radiation at the wavelength corresponding to the laser line (Rayleigh scattering) is filtered out, while the rest of the collected light is dispersed onto a detector by either a notch filter or a band pass filter.

Spontaneous Raman scattering is typically very weak, and as a result the main difficulty of Raman spectroscopy is separating the weak inelastically scattered light from the intense Rayleigh scattered laser light. Historically, Raman spectrometers used holographic gratings and multiple dispersion stages to achieve a high degree of laser rejection. In the past, photomultipliers were the detectors of choice for dispersive Raman setups, which resulted in long acquisition times. However, modern instrumentation almost universally employs notch or edge filters for laser rejection and spectrographs either axial transmissive (AT), Czerny–Turner (CT) monochromator, or FT (Fourier transform spectroscopy based), and CCD detectors.

There are a number of advanced types of Raman spectroscopy, including surface-enhanced Raman, resonance Raman, tip-enhanced Raman, polarised Raman, stimulated Raman (analogous to stimulated emission), transmission Raman, spatially offset Raman, and hyper Raman.

Theoretical basis

The Raman effect occurs when electromagnetic radiation impinges on a molecule and interacts with the polarizable electron density and the bonds of the molecule in the phase (solid, liquid or gaseous) and environment in which the molecule finds itself. For the spontaneous Raman effect, which is a form of inelastic light scattering, a photon (electromagnetic radiation of a specific wavelength) excites (interacts with) the molecule in either the ground rovibronic state (lowest rotational and vibrational energy level of the ground electronic state) or an excited rovibronic state. This results in the molecule being in a so-called virtual energy state for a short period of time before an inelastically scattered photon results. The resulting inelastically scattered photon which is "emitted"/"scattered" can be of either lower (Stokes) or higher (anti-Stokes) energy than the incoming photon. In Raman scattering the resulting rovibronic state of the molecule is a different rotational or vibrational state than the one in which the molecule was originally, before interacting with the incoming photon (electromagnetic radiation). The difference in energy between the original rovibronic state and this resulting rovibronic state leads to a shift in the emitted photon's frequency away from the excitation wavelength, the so-called Rayleigh line. The Raman effect is due to inelastic scattering and should not be confused with emission (fluorescence or phosphorescence) where a molecule in an excited electronic state emits a photon of energy and returns to the ground electronic state, in many cases to a vibrationally excited state on the ground electronic state potential energy surface.

If the final vibrational state of the molecule is more energetic than the initial state, the inelastically scattered photon will be shifted to a lower frequency for the total energy of the system to remain balanced. This shift in frequency is designated as a Stokes shift. If the final vibrational state is less energetic than the initial state, then the inelastically scattered photon will be shifted to a higher frequency, and this is designated as an anti-Stokes shift. Raman scattering is an example of inelastic scattering because of the energy and momentum transfer between the photons and the molecules during the interaction. Rayleigh scattering is an example of elastic scattering, the energy of the scattered Rayleigh scattering is of the same frequency (wavelength) as the incoming electromagnetic radiation.

A change in the molecular electric dipole-electric polarizability with respect to the vibrational coordinate corresponding to the rovibronic state is required for a molecule to exhibit a Raman effect. The intensity of the Raman scattering is proportional to the electric dipole-electric dipole polarizability change. The Raman spectra (Raman scattering intensity as a function of the Stokes and anti-Stokes frequency shifts) is dependent on the rovibronic (rotational and vibrational energy levels of the ground electronic state) states of the sample. This dependence on the electric dipole-electric dipole polarizability derivative differs from infrared spectroscopy where the interaction between the molecule and light is determined by the electric dipole moment derivative, the so-called atomic polar tensor (APT); this contrasting feature allows one to analyze transitions that might not be IR active via Raman spectroscopy, as exemplified by the rule of mutual exclusion in centrosymmetric molecules. Bands which have large Raman intensities in many cases have weak infrared intensities and vice versa. For very symmetric molecules, certain vibrations may be both infrared and Raman inactive (within the harmonic approximation). In those instances, one can use a technique called inelastic incoherent neutron scattering to determine the vibrational frequencies. The selection rules for inelastic incoherent neutron scattering (IINS) are different from those of both infrared and Raman scattering. Hence the three types of vibrational spectroscopy are complementary, all giving in theory the same frequency for a given vibrational transion, but the relative intensities giving different information due to the types of interaction between the molecule and the electromagnetic radiation for infrared and Raman spectroscopy and with the neutron beam for IINS.

History

Although the inelastic scattering of light was predicted by Adolf Smekal in 1923,^[2] it was not until 1928 that it was observed in practice. The Raman effect was named after one of its discoverers, the Indian scientist Sir C. V. Raman who observed the effect by means of sunlight (1928, together with K. S. Krishnan and independently by Grigory Landsberg and Leonid Mandelstam).^[1] Raman won the Nobel Prize in Physics in 1930 for this discovery accomplished using sunlight, a narrow band photographic filter to create monochromatic light, and a "crossed filter" to block this monochromatic light. He found that a small amount of light had changed frequency and passed through the "crossed" filter.

Systematic pioneering theory of the Raman effect was developed by Czechoslovak physicist George Placzek between 1930 and 1934.^[3] The mercury arc became the principal light source, first with photographic detection and then with spectrophotometric detection.

In the years following its discovery, Raman spectroscopy was used to provide the first catalog of molecular vibrational frequencies. Originally, heroic measures were required to obtain Raman spectra due to the low sensitivity of the technique. Typically, the sample was held in a long tube and illuminated along its length with a beam of filtered monochromatic light generated by a gas discharge lamp. The photons that were scattered by the sample were collected through an optical flat at the end of the tube. To maximize the sensitivity, the sample was highly concentrated (1 M or more) and relatively large volumes (5 mL or more) were used. Consequently, the use of Raman spectroscopy dwindled when commercial IR spectrophotometers became available in the 1940s. However, the advent of the laser in the 1960s resulted in simplified Raman spectroscopy instruments and also boosted the sensitivity of the technique. This has revived the use of Raman spectroscopy as a common analytical technique.

Raman shift

Raman shifts are typically reported in wavenumbers, which have units of inverse length, as this value is directly related to energy. In order to convert between spectral wavelength and wavenumbers of shift in the Raman spectrum, the following formula can be used:

where is the Raman shift expressed in wavenumber, λ₀ is the excitation wavelength, and λ₁ is the Raman spectrum wavelength. Most commonly, the unit chosen for expressing wavenumber in Raman spectra is inverse centimeters (cm⁻¹). Since wavelength is often expressed in units of nanometers (nm), the formula above can scale for this unit conversion explicitly, giving

Applications

Raman spectroscopy is commonly used in chemistry, since vibrational information is specific to the chemical bonds and symmetry of molecules. Therefore, it provides a fingerprint by which the molecule can be identified. For instance, the vibrational frequencies of SiO, Si₂O₂, and Si₃O₃ were identified and assigned on the basis of normal coordinate analyses using infrared and Raman spectra.^[4] The fingerprint region of organic molecules is in the (wavenumber) range 500–2000 cm⁻¹. Another way that the technique is used is to study changes in chemical bonding, as when a substrate is added to an enzyme.

Raman gas analyzers have many practical applications. For instance, they are used in medicine for real-time monitoring of anesthetic and respiratory gas mixtures during surgery.

In solid state chemistry and the bio-pharmaceutical industry, Raman spectroscopy can be used to not only identify (ID) active pharmaceutical ingredients (APIs), but in the case of multiple polymorphic forms, it can also be used to identify the polymorphic form of the API. For example there are 4 different polymorphic forms of the API (aztreonam) in Cayston, a drug marketed by Gilead Sciences for cystic fibrosis^{[citation needed]}. Both infrared and Raman spectroscopy can be used to identify and characterize the API which is used in the formulation of Cayston.
In bio-pharmaceutical formulations, one must use not only the correct molecule, but the correct polymorphic form, as different polymorphic forms have different physical properties, for example, solubility, melting point, and Raman/infrared spectra.

In solid-state physics, spontaneous Raman spectroscopy is used to, among other things, characterize materials, measure temperature, and find the crystallographic orientation of a sample. As with single molecules, a given solid material has characteristic phonon modes that can help an experimenter identify it. In addition, Raman spectroscopy can be used to observe other low frequency excitations of the solid, such as plasmons, magnons, and superconducting gap excitations. The spontaneous Raman signal gives information on the population of a given phonon mode in the ratio between the Stokes (downshifted) intensity and anti-Stokes (upshifted) intensity.
Raman scattering by an anisotropic crystal gives information on the crystal orientation. The polarization of the Raman scattered light with respect to the crystal and the polarization of the laser light can be used to find the orientation of the crystal, if the crystal structure (to be specific, its point group) is known.

Raman spectroscopy is the basis for distributed temperature sensing (DTS) along optical fibers, which uses the Raman-shifted backscatter from laser pulses to determine the temperature along optical fibers.

Raman active fibers, such as aramid and carbon, have vibrational modes that show a shift in Raman frequency with applied stress. Polypropylene fibers also exhibit similar shifts. The radial breathing mode is a commonly used technique to evaluate the diameter of carbon nanotubes. In nanotechnology, a Raman microscope can be used to analyze nanowires to better understand the composition of the structures.

Spatially offset Raman spectroscopy (SORS), which is less sensitive to surface layers than conventional Raman, can be used to discover counterfeit drugs without opening their packaging, and for non-invasive monitoring of biological tissue.^[5] Raman spectroscopy can be used to investigate the chemical composition of historical documents such as the Book of Kells and contribute to knowledge of the social and economic conditions at the time the documents were produced.^[6] This is especially helpful because Raman spectroscopy offers a non-invasive way to determine the best course of preservation or conservation treatment for such materials.

Several research projects demonstrated usage of Raman spectroscopy as a means to detect explosives using laser beams from safe distance (Portendo, 2008,^[7] TU Vienna, 2012^[8]).^[9]

Raman spectroscopy has also been used to confirm the prediction of existence of low-frequency phonons ^[10] in proteins and DNA (see, e.g., ^{[11] [12] [13] [14]}) greatly stimulating the studies of low-frequency collective motion in proteins and DNA and their biological functions.^[15][16]

Raman reporter molecules with olefin or alkyne moieties are being developed to allow for tissue imaging with SERS-labeled antibodies.^[17] Raman spectroscopy has also been used as a noninvasive technique for real-time, in situ biochemical characterization of healing wounds and multivariate analysis of Raman spectra has enabled a quantitative measure of wound healing progress.^[18] Raman spectroscopy has a wide usage in studies of biominerals.^[19]

Microspectroscopy

Raman spectroscopy offers several advantages for microscopic analysis. Since it is a scattering technique, specimens do not need to be fixed or sectioned. Raman spectra can be collected from a very small volume (< 1 µm in diameter); these spectra allow the identification of species present in that volume. Water does not generally interfere with Raman spectral analysis. Thus, Raman spectroscopy is suitable for the microscopic examination of minerals, materials such as polymers and ceramics, cells, proteins and forensic trace evidence. A Raman microscope begins with a standard optical microscope, and adds an excitation laser, a monochromator, and a sensitive detector (such as a charge-coupled device (CCD), or photomultiplier tube (PMT)). FT-Raman has also been used with microscopes. Ultraviolet microscopes and UV enhanced optics must be used when a UV laser source is used for Raman microspectroscopy.

In direct imaging, the whole field of view is examined for scattering over a small range of wavenumbers (Raman shifts). For instance, a wavenumber characteristic for cholesterol could be used to record the distribution of cholesterol within a cell culture.

The other approach is hyperspectral imaging or chemical imaging, in which thousands of Raman spectra are acquired from all over the field of view. The data can then be used to generate images showing the location and amount of different components. Taking the cell culture example, a hyperspectral image could show the distribution of cholesterol, as well as proteins, nucleic acids, and fatty acids. Sophisticated signal- and image-processing techniques can be used to ignore the presence of water, culture media, buffers, and other interference.

Raman microscopy, and in particular confocal microscopy, has very high spatial resolution. For example, the lateral and depth resolutions were 250 nm and 1.7 µm, respectively, using a confocal Raman microspectrometer with the 632.8 nm line from a helium–neon laser with a pinhole of 100 µm diameter. Since the objective lenses of microscopes focus the laser beam to several micrometres in diameter, the resulting photon flux is much higher than achieved in conventional Raman setups. This has the added benefit of enhanced fluorescence quenching. However, the high photon flux can also cause sample degradation, and for this reason some setups require a thermally conducting substrate (which acts as a heat sink) in order to mitigate this process.

Another approach called global Raman imaging^[20] uses complete monochromatic images instead of reconstruction of images from acquired spectra. This technique is being used for the characterization of large scale devices, mapping of different compounds and dynamics study. It has already been use for the characterization of graphene layers,^[21] J-aggregated dyes inside carbon nanotubes^[22] and multiple other 2D materials such as MoS2 and WSe2. Since the excitation beam is dispersed over the whole field of view, those measurements can be done without damaging the sample.

By using Raman microspectroscopy, in vivo time- and space-resolved Raman spectra of microscopic regions of samples can be measured. As a result, the fluorescence of water, media, and buffers can be removed. Consequently in vivo time- and space-resolved Raman spectroscopy is suitable to examine proteins, cells and organs.

Raman microscopy for biological and medical specimens generally uses near-infrared (NIR) lasers (785 nm diodes and 1064 nm Nd:YAG are especially common). This reduces the risk of damaging the specimen by applying higher energy wavelengths. However, the intensity of NIR Raman is low (owing to the ω⁴ dependence of Raman scattering intensity), and most detectors require very long collection times. Recently, more sensitive detectors have become available, making the technique better suited to general use. Raman microscopy of inorganic specimens, such as rocks and ceramics and polymers, can use a broader range of excitation wavelengths.^[23]

Polarized analysis

The polarization of the Raman scattered light also contains useful information. This property can be measured using (plane) polarized laser excitation and a polarization analyzer. Spectra acquired with the analyzer set at both perpendicular and parallel to the excitation plane can be used to calculate the depolarization ratio. Study of the technique is useful in teaching the connections between group theory, symmetry, Raman activity, and peaks in the corresponding Raman spectra.^[24] Polarized light only gives access to some of the Raman active modes. By rotating the polarization you can gain access to the other modes. Each mode is separated according to its symmetry.^[25]

The spectral information arising from this analysis gives insight into molecular orientation and vibrational symmetry. In essence, it allows the user to obtain valuable information relating to the molecular shape, for example in synthetic chemistry or polymorph analysis. It is often used to understand macromolecular orientation in crystal lattices, liquid crystals or polymer samples.^[26]

It is convenient in polarised Raman spectroscopy to describe the propagation and polarisation directions using Porto's notation,^[27] described by and named after Brazilian physicist Sergio Pereira da Silva Porto.

Variations

Several variations of Raman spectroscopy have been developed. The usual purpose is to enhance the sensitivity (e.g., surface-enhanced Raman), to improve the spatial resolution (Raman microscopy), or to acquire very specific information (resonance Raman).

• Surface-enhanced Raman spectroscopy (SERS) – Normally done in a silver or gold colloid or a substrate containing silver or gold. Surface plasmons of silver and gold are excited by the laser, resulting in an increase in the electric fields surrounding the metal. Given that Raman intensities are proportional to the electric field, there is large increase in the measured signal (by up to 10¹¹). This effect was originally observed by Martin Fleischmann but the prevailing explanation was proposed by Van Duyne in 1977.^[28] A comprehensive theory of the effect was given by Lombardi and Birke.^[29]
• Resonance Raman spectroscopy – The excitation wavelength is matched to an electronic transition of the molecule or crystal, so that vibrational modes associated with the excited electronic state are greatly enhanced. This is useful for studying large molecules such as polypeptides, which might show hundreds of bands in "conventional" Raman spectra. It is also useful for associating normal modes with their observed frequency shifts.^[30]
• Surface-enhanced resonance Raman spectroscopy (SERRS) – A combination of SERS and resonance Raman spectroscopy that uses proximity to a surface to increase Raman intensity, and excitation wavelength matched to the maximum absorbance of the molecule being analysed.
• Angle-resolved Raman spectroscopy – Not only are standard Raman results recorded but also the angle with respect to the incident laser. If the orientation of the sample is known then detailed information about the phonon dispersion relation can also be gleaned from a single test.^[31]
• Hyper Raman – A non-linear effect in which the vibrational modes interact with the second harmonic of the excitation beam. This requires very high power, but allows the observation of vibrational modes that are normally "silent". It frequently relies on SERS-type enhancement to boost the sensitivity.^[32]
• Spontaneous Raman spectroscopy (SRS) – Used to study the temperature dependence of the Raman spectra of molecules.
• Optical tweezers Raman spectroscopy (OTRS) – Used to study individual particles, and even biochemical processes in single cells trapped by optical tweezers.
• Stimulated Raman spectroscopy – A spatially coincident, two color pulse (with polarization either parallel or perpendicular) transfers the population from ground to a rovibrationally excited state, if the difference in energy corresponds to an allowed Raman transition, and if neither frequency corresponds to an electronic resonance. Two photon UV ionization, applied after the population transfer but before relaxation, allows the intra-molecular or inter-molecular Raman spectrum of a gas or molecular cluster (indeed, a given conformation of molecular cluster) to be collected. This is a useful molecular dynamics technique.
• Spatially offset Raman spectroscopy (SORS) – The Raman scattering beneath an obscuring surface is retrieved from a scaled subtraction of two spectra taken at two spatially offset points
• Coherent anti-Stokes Raman spectroscopy (CARS) – Two laser beams are used to generate a coherent anti-Stokes frequency beam, which can be enhanced by resonance.
• Raman optical activity (ROA) – Measures vibrational optical activity by means of a small difference in the intensity of Raman scattering from chiral molecules in right- and left-circularly polarized incident light or, equivalently, a small circularly polarized component in the scattered light.^[33]
• Transmission Raman – Allows probing of a significant bulk of a turbid material, such as powders, capsules, living tissue, etc. It was largely ignored following investigations in the late 1960s (Schrader and Bergmann, 1967)^[34] but was rediscovered in 2006 as a means of rapid assay of pharmaceutical dosage forms.^[35] There are medical diagnostic applications particularly in the detection of cancer.^[9][36][37]
• Inverse Raman spectroscopy.
• Tip-enhanced Raman spectroscopy (TERS) – Uses a metallic (usually silver-/gold-coated AFM or STM) tip to enhance the Raman signals of molecules situated in its vicinity. The spatial resolution is approximately the size of the tip apex (20–30 nm). TERS has been shown to have sensitivity down to the single molecule level and holds some promise for bioanalysis applications.^[38]
• Surface plasmon polariton enhanced Raman scattering (SPPERS) – This approach exploits apertureless metallic conical tips for near field excitation of molecules. This technique differs from the TERS approach due to its inherent capability of suppressing the background field. In fact, when an appropriate laser source impinges on the base of the cone, a TM0 mode ^[39] (polaritonic mode) can be locally created, namely far away from the excitation spot (apex of the tip). The mode can propagate along the tip without producing any radiation field up to the tip apex where it interacts with the molecule. In this way, the focal plane is separated from the excitation plane by a distance given by the tip length, and no background plays any role in the Raman excitation of the molecule.^{[40][41][42][43]}
• Micro-Cavity Substrates – A method that improves the detection limit of conventional Raman spectra using micro-Raman in a micro-cavity coated with reflective Au or Ag. The micro-cavity has a radius of several micrometers and enhances the entire Raman signal by providing multiple excitations of the sample and couples the forward-scattered Raman photons toward the collection optics in the back-scattered Raman geometry.^[44]
• Stand-off Remote Raman – Standoff Raman detection offers a fast-Raman mode of analyzing large areas such as a football field in minutes. A pulsed laser source and gated detector allow Raman spectra measurements in the daylight and reduces the long-lived fluorescent background generated by transition ions and rare earth ions. Another way to avoid fluorescence, first demonstrated by Sandy Asher in 1984, is to use a UV laser probe beam. At wavelengths of 260 nm, there is effectively no fluorescence interference and the UV signal is inherently strong.^[9][45][46] A 10X beam expander mounted in front of the laser allows focusing of the beam and a telescope is directly coupled through the camera lens for signal collection. With the system's time-gating capability it is possible to measure remote Raman of your distant target and the atmosphere between the laser and target.^[9]