Optical properties of carbon nanotubes

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Optical_properties_of_carbon_nanotubes

 

A sample of multiwalled carbon nanotubes with 3–15 walls, mean inner diameter 4 nm, mean outer diameter 13–16 nm, length 1-10+ micrometers.

 

The optical properties of carbon nanotubes are highly relevant for materials science. The way those materials interact with electromagnetic radiation is unique in many respects, as evidenced by their peculiar absorption, photoluminescence (fluorescence), and Raman spectra.

Carbon nanotubes are unique "one-dimensional" materials, whose hollow fibers (tubes) have a unique and highly ordered atomic and electronic structure, and can be made in a wide range of dimension. The diameter typically varies from 0.4 to 40 nm (i.e., a range of ~100 times). However, the length can reach 55.5 cm (21.9 in), implying a length-to-diameter ratio as high as 132,000,000:1; which is unequaled by any other material. Consequently, all the electronic, optical, electrochemical and mechanical properties of the carbon nanotubes are extremely anisotropic (directionally dependent) and tunable.

Applications of carbon nanotubes in optics and photonics are still less developed than in other fields. Some properties that may lead to practical use include tuneability and wavelength selectivity. Potential applications that have been demonstrated include light emitting diodes (LEDs), bolometers and optoelectronic memory.

Apart from direct applications, the optical properties of carbon nanotubes can be very useful in their manufacture and application to other fields. Spectroscopic methods offer the possibility of quick and non-destructive characterization of relatively large amounts of carbon nanotubes, yielding detailed measurements of non-tubular carbon content, tube type and chirality, structural defects, and many other properties that are relevant to those other applications.

Geometric structure

Main article: Carbon nanotube § Structure

Chiral angle

A single-walled carbon nanotubes (SWCNT) can be envisioned as strip of a graphene molecule (a single sheet of graphite) rolled and joined into a seamless cylinder. The structure of the nanotube can be characterized by the width of this hypothetical strip (that is, the circumference c or diameter d of the tube) and the angle α of the strip relative to the main symmetry axes of the hexagonal graphene lattice. This angle, which may vary from 0 to 30 degrees, is called the "chiral angle" of the tube.

The (n,m) notation

A "sliced and unrolled" representation of a carbon nanotube as a strip of a graphene molecule, overlaid on a diagram of the full molecule (faint background). The vector w (large blue arrow) connects corresponding positions on the two edges of the strip. Since w = 3u + 1v, the tube is said to be of type (3,1).

Alternatively, the structure can be described by two integer indices (n,m) that describe the width and direction of that hypothetical strip as coordinates in a fundamental reference frame of the graphene lattice. If the atoms around any 6-member ring of the graphene are numbered sequentially from 1 to 6, the two vectors u and v of that frame are the displacements from atom 1 to atoms 3 and 5, respectively. Those two vectors have the same length, and their directions are 60 degrees apart. The vector w = n u + m v is then interpreted as the circumference of the unrolled tube on the graphene lattice; it relates each point A1 on one edge of the strip to the point A2 on the other edge that will be identified with it as the strip is rolled up. The chiral angle α is then the angle between u and w.

The pairs (n,m) that describe distinct tube structures are those with 0 ≤ m ≤ n and n > 0. All geometric properties of the tube, such as diameter, chiral angle, and symmetries, can be computed from these indices.

The type also determines the electronic structure of the tube. Specifically, the tube behaves like a metal if |m–n| is a multiple of 3, and like a semiconductor otherwise.

Zigzag and armchair tubes

Tubes of type (n,m) with n=m (chiral angle = 30°) are called "armchair" and those with m=0 (chiral angle = 0°) "zigzag". These tubes have mirror symmetry, and can be viewed as stacks of simple closed paths ("zigzag" and "armchair" paths, respectively).

Armchair nanotube

Zigzag nanotube

Electronic structure

The optical properties of carbon nanotubes are largely determined by their unique electronic structure. The rolling up of the graphene lattice affects that structure in ways that depend strongly on the geometric structure type (n,m).

Van Hove singularities

A bulk 3D material (blue) has continuous DOS, but a 1D wire (green) has Van Hove singularities.

A characteristic feature of one-dimensional crystals is that their distribution of density of states (DOS) is not a continuous function of energy, but it descends gradually and then increases in a discontinuous spike. These sharp peaks are called Van Hove singularities. In contrast, three-dimensional materials have continuous DOS.

Van Hove singularities result in the following remarkable optical properties of carbon nanotubes:

• Optical transitions occur between the v₁ − c₁, v₂ − c₂, etc., states of semiconducting or metallic nanotubes and are traditionally labeled as S₁₁, S₂₂, M₁₁, etc., or, if the "conductivity" of the tube is unknown or unimportant, as E₁₁, E₂₂, etc. Crossover transitions c₁ − v₂, c₂ − v₁, etc., are dipole-forbidden and thus are extremely weak, but they were possibly observed using cross-polarized optical geometry.

• The energies between the Van Hove singularities depend on the nanotube structure. Thus by varying this structure, one can tune the optoelectronic properties of carbon nanotube. Such fine tuning has been experimentally demonstrated using UV illumination of polymer-dispersed CNTs.

• Optical transitions are rather sharp (~10 meV) and strong. Consequently, it is relatively easy to selectively excite nanotubes having certain (n, m) indices, as well as to detect optical signals from individual nanotubes.

Kataura plot

In this Kataura plot, the energy of an electronic transition decreases as the diameter of the nanotube increases.

The band structure of carbon nanotubes having certain (n, m) indexes can be easily calculated. A theoretical graph based on these calculations was designed in 1999 by Hiromichi Kataura to rationalize experimental findings. A Kataura plot relates the nanotube diameter and its bandgap energies for all nanotubes in a diameter range. The oscillating shape of every branch of the Kataura plot reflects the intrinsic strong dependence of the SWNT properties on the (n, m) index rather than on its diameter. For example, (10, 1) and (8, 3) tubes have almost the same diameter, but very different properties: the former is a metal, but the latter is a semiconductor.

Optical properties

Optical absorption

Optical absorption spectrum from dispersed single-wall carbon nanotubes

Optical absorption in carbon nanotubes differs from absorption in conventional 3D materials by presence of sharp peaks (1D nanotubes) instead of an absorption threshold followed by an absorption increase (most 3D solids). Absorption in nanotubes originates from electronic transitions from the v₂ to c₂ (energy E₂₂) or v₁ to c₁ (E₁₁) levels, etc. The transitions are relatively sharp and can be used to identify nanotube types. Note that the sharpness deteriorates with increasing energy, and that many nanotubes have very similar E₂₂ or E₁₁ energies, and thus significant overlap occurs in absorption spectra. This overlap is avoided in photoluminescence mapping measurements (see below), which instead of a combination of overlapped transitions identifies individual (E₂₂, E₁₁) pairs.

Interactions between nanotubes, such as bundling, broaden optical lines. While bundling strongly affects photoluminescence, it has much weaker effect on optical absorption and Raman scattering. Consequently, sample preparation for the latter two techniques is relatively simple.

Optical absorption is routinely used to quantify quality of the carbon nanotube powders.

The spectrum is analyzed in terms of intensities of nanotube-related peaks, background and pi-carbon peak; the latter two mostly originate from non-nanotube carbon in contaminated samples. However, it has been recently shown that by aggregating nearly single chirality semiconducting nanotubes into closely packed Van der Waals bundles the absorption background can be attributed to free carrier transition originating from intertube charge transfer.

Carbon nanotubes as a black body

An ideal black body should have emissivity or absorbance of 1.0, which is difficult to attain in practice, especially in a wide spectral range. Vertically aligned "forests" of single-wall carbon nanotubes can have absorbances of 0.98–0.99 from the far-ultraviolet (200 nm) to far-infrared (200 μm) wavelengths.

These SWNT forests (buckypaper) were grown by the super-growth CVD method to about 10 μm height. Two factors could contribute to strong light absorption by these structures: (i) a distribution of CNT chiralities resulted in various bandgaps for individual CNTs. Thus a compound material was formed with broadband absorption. (ii) Light might be trapped in those forests due to multiple reflections.

Reflectance measurements

	UV-to-near IR	Near-to-mid IR	Mid-to-far IR
Wavelength, μm	0.2-2	2–20	25–200
Incident angle, °	8	5	10
Reflection	Hemispherical-directional	Hemispherical-directional	Specular
Reference	White reflectance standard	Gold mirror	Aluminum mirror
Average reflectance	0.0160	0.0097	0.0017
Standard deviation	0.0048	0.0041	0.0027

Luminescence

Photoluminescence map from single-wall carbon nanotubes. (n, m) indexes identify certain semiconducting nanotubes. Note that PL measurements do not detect nanotubes with n = m or m = 0.

Photoluminescence (fluorescence)

Semiconducting single-walled carbon nanotubes emit near-infrared light upon photoexcitation, described interchangeably as fluorescence or photoluminescence (PL). The excitation of PL usually occurs as follows: an electron in a nanotube absorbs excitation light via S₂₂ transition, creating an electron-hole pair (exciton). Both electron and hole rapidly relax (via phonon-assisted processes) from c₂ to c₁ and from v₂ to v₁ states, respectively. Then they recombine through a c₁ − v₁ transition resulting in light emission.

No excitonic luminescence can be produced in metallic tubes. Their electrons can be excited, thus resulting in optical absorption, but the holes are immediately filled by other electrons out of the many available in the metal. Therefore, no excitons are produced.

Salient properties

• Photoluminescence from SWNT, as well as optical absorption and Raman scattering, is linearly polarized along the tube axis. This allows monitoring of the SWNTs orientation without direct microscopic observation.
• PL is quick: relaxation typically occurs within 100 picoseconds.
• PL efficiency was first found to be low (~0.01%), but later studies measured much higher quantum yields. By improving the structural quality and isolation of nanotubes, emission efficiency increased. A quantum yield of 1% was reported in nanotubes sorted by diameter and length through gradient centrifugation, and it was further increased to 20% by optimizing the procedure of isolating individual nanotubes in solution.
• The spectral range of PL is rather wide. Emission wavelength can vary between 0.8 and 2.1 micrometers depending on the nanotube structure.
• Excitons are apparently delocalized over several nanotubes in single chirality bundles as the photoluminescence spectrum displays a splitting consistent with intertube exciton tunneling.
• Interaction between nanotubes or between a nanotube and another material may quench or increase PL. No PL is observed in multi-walled carbon nanotubes. PL from double-wall carbon nanotubes strongly depends on the preparation method: CVD grown DWCNTs show emission both from inner and outer shells. However, DWCNTs produced by encapsulating fullerenes into SWNTs and annealing show PL only from the outer shells. Isolated SWNTs lying on the substrate show extremely weak PL which has been detected in few studies only. Detachment of the tubes from the substrate drastically increases PL.

• Position of the (S₂₂, S₁₁) PL peaks depends slightly (within 2%) on the nanotube environment (air, dispersant, etc.). However, the shift depends on the (n, m) index, and thus the whole PL map not only shifts, but also warps upon changing the CNT medium.

Raman scattering

Main article: Raman spectroscopy

 

Raman spectrum of single-wall carbon nanotubes

Raman spectroscopy has good spatial resolution (~0.5 micrometers) and sensitivity (single nanotubes); it requires only minimal sample preparation and is rather informative. Consequently, Raman spectroscopy is probably the most popular technique of carbon nanotube characterization. Raman scattering in SWNTs is resonant, i.e., only those tubes are probed which have one of the bandgaps equal to the exciting laser energy. Several scattering modes dominate the SWNT spectrum, as discussed below.

Similar to photoluminescence mapping, the energy of the excitation light can be scanned in Raman measurements, thus producing Raman maps. Those maps also contain oval-shaped features uniquely identifying (n, m) indices. Contrary to PL, Raman mapping detects not only semiconducting but also metallic tubes, and it is less sensitive to nanotube bundling than PL. However, requirement of a tunable laser and a dedicated spectrometer is a strong technical impediment.

Radial breathing mode

Radial breathing mode (RBM) corresponds to radial expansion-contraction of the nanotube. Therefore, its frequency ν_RBM (in cm⁻¹) depends on the nanotube diameter d as, ν_RBM= A/d + B (where A and B are constants dependent on the environment in which the nanotube is present. For example, B=0 for individual nanotubes.) (in nanometers) and can be estimated as ν_RBM = 234/d + 10 for SWNT or ν_RBM = 248/d for DWNT, which is very useful in deducing the CNT diameter from the RBM position. Typical RBM range is 100–350 cm⁻¹. If RBM intensity is particularly strong, its weak second overtone can be observed at double frequency.

Bundling mode

The bundling mode is a special form of RBM supposedly originating from collective vibration in a bundle of SWNTs.

G mode

Another very important mode is the G mode (G from graphite). This mode corresponds to planar vibrations of carbon atoms and is present in most graphite-like materials. G band in SWNT is shifted to lower frequencies relative to graphite (1580 cm⁻¹) and is split into several peaks. The splitting pattern and intensity depend on the tube structure and excitation energy; they can be used, though with much lower accuracy compared to RBM mode, to estimate the tube diameter and whether the tube is metallic or semiconducting.

D mode

D mode is present in all graphite-like carbons and originates from structural defects. Therefore, the ratio of the G/D modes is conventionally used to quantify the structural quality of carbon nanotubes. High-quality nanotubes have this ratio significantly higher than 100. At a lower functionalisation of the nanotube, the G/D ratio remains almost unchanged. This ratio gives an idea of the functionalisation of a nanotube.

G' mode

The name of this mode is misleading: it is given because in graphite, this mode is usually the second strongest after the G mode. However, it is actually the second overtone of the defect-induced D mode (and thus should logically be named D'). Its intensity is stronger than that of the D mode due to different selection rules. In particular, D mode is forbidden in the ideal nanotube and requires a structural defect, providing a phonon of certain angular momentum, to be induced. In contrast, G' mode involves a "self-annihilating" pair of phonons and thus does not require defects. The spectral position of G' mode depends on diameter, so it can be used roughly to estimate the SWNT diameter. In particular, G' mode is a doublet in double-wall carbon nanotubes, but the doublet is often unresolved due to line broadening.

Other overtones, such as a combination of RBM+G mode at ~1750 cm⁻¹, are frequently seen in CNT Raman spectra. However, they are less important and are not considered here.

Anti-Stokes scattering

All the above Raman modes can be observed both as Stokes and anti-Stokes scattering. As mentioned above, Raman scattering from CNTs is resonant in nature, i.e. only tubes whose band gap energy is similar to the laser energy are excited. The difference between those two energies, and thus the band gap of individual tubes, can be estimated from the intensity ratio of the Stokes/anti-Stokes lines. This estimate however relies on the temperature factor (Boltzmann factor), which is often miscalculated – a focused laser beam is used in the measurement, which can locally heat the nanotubes without changing the overall temperature of the studied sample.

Rayleigh scattering

Carbon nanotubes have very large aspect ratio, i.e., their length is much larger than their diameter. Consequently, as expected from the classical electromagnetic theory, elastic light scattering (or Rayleigh scattering) by straight CNTs has anisotropic angular dependence, and from its spectrum, the band gaps of individual nanotubes can be deduced.

Another manifestation of Rayleigh scattering is the "antenna effect", an array of nanotubes standing on a substrate has specific angular and spectral distributions of reflected light, and both those distributions depend on the nanotube length.

Applications

Light emitting diodes (LEDs) and photo-detectors based on a single nanotube have been produced in the lab. Their unique feature is not the efficiency, which is yet relatively low, but the narrow selectivity in the wavelength of emission and detection of light and the possibility of its fine tuning through the nanotube structure. In addition, bolometer and optoelectronic memory devices have been realised on ensembles of single-walled carbon nanotubes.

• Photoluminescence is used for characterization purposes to measure the quantities of semiconducting nanotube species in a sample. Nanotubes are isolated (dispersed) using an appropriate chemical agent ("dispersant") to reduce the intertube quenching. Then PL is measured, scanning both the excitation and emission energies and thereby producing a PL map. The ovals in the map define (S₂₂, S₁₁) pairs, which unique identify (n, m) index of a tube. The data of Weisman and Bachilo are conventionally used for the identification.

• Nanotube fluorescence has been investigated for the purposes of imaging and sensing in biomedical applications.

Sensitization

Optical properties, including the PL efficiency, can be modified by encapsulating organic dyes (carotene, lycopene, etc.) inside the tubes. Efficient energy transfer occurs between the encapsulated dye and nanotube — light is efficiently absorbed by the dye and without significant loss is transferred to the SWNT. Thus potentially, optical properties of a carbon nanotube can be controlled by encapsulating certain molecule inside it. Besides, encapsulation allows isolation and characterization of organic molecules which are unstable under ambient conditions. For example, Raman spectra are extremely difficult to measure from dyes because of their strong PL (efficiency close to 100%). However, encapsulation of dye molecules inside SWNTs completely quenches dye PL, thus allowing measurement and analysis of their Raman spectra.

Cathodoluminescence

Cathodoluminescence (CL) — light emission excited by electron beam — is a process commonly observed in TV screens. An electron beam can be finely focused and scanned across the studied material. This technique is widely used to study defects in semiconductors and nanostructures with nanometer-scale spatial resolution. It would be beneficial to apply this technique to carbon nanotubes. However, no reliable CL, i.e. sharp peaks assignable to certain (n, m) indices, has been detected from carbon nanotubes yet.

Electroluminescence

If appropriate electrical contacts are attached to a nanotube, electron-hole pairs (excitons) can be generated by injecting electrons and holes from the contacts. Subsequent exciton recombination results in electroluminescence (EL). Electroluminescent devices have been produced from single nanotubes and their macroscopic assemblies. Recombination appears to proceed via triplet-triplet annihilation giving distinct peaks corresponding to E₁₁ and E₂₂ transitions.

Multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWNT) may consist of several nested single-walled tubes, or of a single graphene strip rolled up multiple times, like a scroll. They are difficult to study because their properties are determined by contributions and interactions of all individual shells, which have different structures. Moreover, the methods used to synthesize them are poorly selective and result in higher incidence of defects.

Carbon nanotube chemistry

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Carbon_nanotube_chemistry

Carbon nanotube chemistry involves chemical reactions, which are used to modify the properties of carbon nanotubes (CNTs). CNTs can be functionalized to attain desired properties that can be used in a wide variety of applications. The two main methods of CNT functionalization are covalent and non-covalent modifications.

Because of their hydrophobic nature, CNTs tend to agglomerate hindering their dispersion in solvents or viscous polymer melts. The resulting nanotube bundles or aggregates reduce the mechanical performance of the final composite. The surface of CNTs can be modified to reduce the hydrophobicity and improve interfacial adhesion to a bulk polymer through chemical attachment.

Chart summarizing options for the chemical modification of carbon nanotubes.

Covalent modification

Covalent modification of carbon nanotubes.

Covalent modification attaches a functional group onto the carbon nanotube. The functional groups can be attached onto the side wall or ends of the carbon nanotube. The end caps of the carbon nanotubes have the highest reactivity due to its higher pyrimidization angle and the walls of the carbon nanotubes have lower pyrimidization angles which has lower reactivity. Although covalent modifications are very stable, the bonding process disrupts the sp² hybridization of the carbon atoms because a σ-bond is formed. The disruption of the extended sp² hybridization typically decreases the conductance of the carbon nanotubes.

Oxidation

The purification and oxidation of carbon nanotubes (CNTs) has been well represented in literature. These processes were essential for low yield production of carbon nanotubes where carbon particles, amorphous carbon particles and coatings comprised a significant percentage of the overall material and are still important for the introduction of surface functional groups. During acid oxidation, the carbon-carbon bonded network of the graphitic layers is broken allowing the introduction of oxygen units in the form of carboxyl, phenolic and lactone groups, which have been extensively exploited for further chemical functionalisation.

First studies on oxidation of carbon nanotubes involved a gas-phase reactions with nitric acid vapor in air, which indiscriminately functionalized the carbon nanotubes with carboxylic, carbonyl or hydroxyl groups. In liquid-phase reactions, carbon nanotubes were treated with oxidizing solutions of nitric acid or a combination of nitric and sulfuric acid to the same effect. However, overoxidation may occur causing the carbon nanotube to break up into fragments, which are known as carbonaceous fragments. Xing et al. revealed sonication assisted oxidation, with sulfuric and nitric acid, of carbon nanotubes and produced carbonyl and carboxyl groups. After the oxidation reaction in acidic solution, treatment with hydrogen peroxide limited the damage on the carbon nanotube network. Single-walled carbon nanotubes can be shortened in a scalable manner using oleum (100% H₂SO₄ with 3% SO₃) and nitric acid. The nitric acid cuts carbon nanotubes while the oleum creates a channel.

In one type of chemical modification, aniline is oxidized to a diazonium intermediate. After expulsion of nitrogen, it forms a covalent bond as an aryl radical:

Esterification/Amidation

Carboxylic groups are used as the precursor for most esterification and amidation reactions. The carboxylic group is converted into an acyl chloride with the use of thionyl or oxalyl chloride which is then reacted with the desired amide, amine, or alcohol. Carbon nanotubes have been deposited on with silver nanoparticles with the aid of amination reactions. Amide functionalized carbon nanotubes have been shown to chelate silver nanoparticles. Carbon nanotubes modified with acyl chloride react readily with highly branched molecules such as poly(amidoamine), which acts as a template for silver ion and later being reduced by formaldehyde. Amino-modified carbon nanotubes can be prepared by reacting ethylenediamine with an acyl chloride functionalized carbon nanotubes.

Halogenation reactions

Carbon nanotubes can be treated with peroxytrifluroacetic acid to give mainly carboxylic acid and trifluroacetic functional groups. The fluorinated carbon nanotubes, through substitution, can be further functionalized with urea, guanidine, thiourea and aminosilane. Using the Hunsdiecker reaction, carbon nanotubes treated with nitric acid can react with iodosobenzenediacetate to iodate carbon nanotubes.

Cycloaddition

Also known are protocols for cycloadditions such as Diels-Alder reactions, 1,3-dipolar cycloadditions of azomethine ylides and azide–alkyne cycloaddition reactions. One example is a DA reaction assisted by chromium hexacarbonyl and high pressure. The I_D/I_G ratio for reaction with Danishefsky's diene is 2.6.

The most well-known 1,3 cycloaddition reaction involves azomethine ylides reacting with carbon nanotubes, which are of great interest. The addition of a pyrrolidine ring can lead to a variety of functional groups such as second-generation poly(amidoamine) dendrimers, phthalocyanine addends, perfluoroalkylsilane groups, and amino ethyleneglycol groups. The Diels-cycloaddition reaction can occur, especially on fluorinated carbon nanotubes. They are known to undergo Diels–Alder reactions with dienes such as 2,3-dimethyl-1,3-butadiene, anthracene, and 2-trimethylsiloxyl-1,3-butadiene.

Radical addition

Top: electron micrographs showing interaction of CNTs with 4-(1-pyrenyl)phenyl radical (a) and its boronic ester (b). Bottom: corresponding models.

The modification of carbon nanotubes with aryl diazonium salts was studied first by Tour et al. Due to the harsh conditions needed for the in situ generated diazonium compound, other methods have been explored. Stephenson et al. reported using aniline derivatives with sodium nitrite in 96% sulfuric acid and ammonium persulfate. Price et al. demonstrated that stirring carbon nanotubes in water and treating with anilines and oxidizing agents proved to be a milder reaction. The diazonium chemistry functionalized carbon nanotubes which was used as a precursor to further modifications. Suzuki and Heck coupling reactions were performed on iodophenyl-functionalized carbon nanotubes. Wong et al. demonstrated mild photochemical reactions to silylate the carbon nanotubes with trimethoxysilane and hexaphenyldisilane.

Nucleophilic addition

Hirsch et al. conducted nucleophilic additions with organolithium and organomagnesium compounds onto carbon nanotubes. With further oxidation in air, they were able to create alkyl-modified carbon nanotubes. Hirsch was also able to show the nucleophilic addition of amines by generating lithium amides, leading to amino-modified carbon nanotubes.

Electrophilic addition

Nanotubes can also be alkylated with alkyl halides using lithium or sodium metal and liquid ammonia (Birch reduction conditions). The initial nanotube salt can function as a polymerization initiator and can react with peroxides to form alkoxy functionalized nanotubes. 

The alkyl and hydroxyl modification of carbon nanotubes was demonstrated with the electrophilic addition of alkylhalides by microwave irradiation. Tessonnier et al. modified carbon nanotubes with amino groups by deprotonating with butyl lithium and reacting with amino substitution. Balaban et al. applied Friedel-Crafts acylation to carbon nanotubes with nitrobenzene at 180 °C along with aluminum chloride.

Non-covalent modifications

Non-covalent modification of carbon nanotubes.

Non-covalent modifications utilize van der Waals forces and π-π interactions by adsorption of polynuclear aromatic compounds, surfactants, polymers or biomolecules. Non-covalent modifications do not disrupt the natural configuration of carbon nanotubes with the cost of chemical stability, and is prone to phase separation, dissociation in between two phases, in the solid state.

Polynuclear aromatic compounds

Some common polynuclear aromatic compounds that are functionalized with hydrophilic or hydrophobic moieties are used to solubilize carbon nanotubes into organic or aqueous solvents. Some of these amphiphiles are phenyl, naphthalene, phenanthrene, pyrene and porphyrin systems. The greater π-π stacking of aromatic amphiphiles such as pyrene amphiphiles had the best solubility compared to phenyl amphiphiles with the worse π-π stacking, lead to more solubility in water. These aromatic systems can be modified with amino and carboxylic acid groups prior to functionalizing the carbon nanotubes.

Biomolecules

The interaction between carbon nanotubes and biomolecules has been widely studied because of their potential to be used in biological applications. The modification of the carbon nanotubes with proteins, carbohydrates, and nucleic acids are built with the bottom-up technique. Proteins have high affinity to carbon nanotubes due to their diversity of amino acids being hydrophobic or hydrophilic. Polysaccharides have been successfully been used to modify carbon nanotubes forming stable hybrids. To make carbon nanotubes soluble in water, phospholipids such as lysoglycerophospholipids have been used. The single phospholipid tail wraps around the carbon nanotube, but the double tailed phospholipids did not have the same ability.

π-π stacking and electrostatic interactions

Molecules that have bifunctionality are used to modify the carbon nanotube. One end of the molecule are polyaromatic compounds that interact with the carbon nanotube through π-π stacking. The other end of the same molecule has a functional group such as amino, carboxyl, or thiol. For example, pyrene derivatives and aryl thiols were used as the linkers for various metal nanobeads such as gold, silver and platinum.

Mechanical interlocking

A particular case of non-covalent modification is the formation of rotaxane-like mechanically interlocked derivatives of single-walled nanotubes (SWNTs). In this strategy, the SWNTs are encapsulated by molecular macrocycle(s), which are either formed around them by macrocyclization, or pre-formed and threaded at a later stage. In MINTs (Mechanically Interlocked NanoTubes), the SWNT and organic macrocycle are linked by means of their topology, through a mechanical bond, combining the stability of the covalent strategies -at least one covalent bond must be broken to separate SWNT and macrocycle(s)- with the structural integrity of the classic noncovalent strategies -the C-sp² network of the SWNT remains intact.

Characterization

A useful tool for the analysis of derivatised nanotubes is Raman spectroscopy which shows a G-band (G for graphite) for the native nanotubes at 1580 cm⁻¹ and a D-band (D for defect) at 1350  cm⁻¹ when the graphite lattice is disrupted with conversion of sp² to sp³ hybridized carbon. The ratio of both peaks I_D/I_G is taken as a measure of functionalization. Other tools are UV spectroscopy where pristine nanotubes show distinct Van Hove singularities where functionalized tubes do not, and simple TGA analysis.

Geography of the United States

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Geography_of_the_United_States 

Political map of the United States showing the 50 states, the District of Columbia and the 5 major U.S. territories.

Geography of The United States

Continent	North America
Coordinates	38.000°N 97.000°W
Area	Ranked 3rd/4th
• Total	9,826,675 km2 (3,794,100 sq mi)
• Land	93.24%
• Water	6.76%
Coastline	19,920 km (12,380 mi)
Borders	Canada: 8,864 km (5,508 mi) Mexico: 3,327 km (2,067 mi)
Highest point	Denali 6,190.5 m (20,310 ft)
Lowest point	Badwater Basin, −85 m (−279 ft)
Longest river	Missouri River, 3,767 km (2,341 mi)
Largest lake	Lake Superior 58,000 km2 (22,394 sq mi)
Climate	Diverse: Ranges from Temperate in the North to Tropical in the far south. West: mostly semi-arid to desert, Mountains: alpine, Northeast: humid continental, Southeast: humid subtropical, Coast of California: Mediterranean, Pacific Northwest: cool temperate oceanic, Alaska: mostly subarctic, Hawaii, South Florida, and the territories: tropical
Terrain	Vast central plain, Interior Highlands and low mountains in Midwest, mountains and valleys in the mid-south, coastal flatland near the Gulf and Atlantic coasts, complete with mangrove forests and temperate, subtropical, and tropical laurel forest and jungle, canyons, basins, plateaus, and mountains in west, hills and low mountains in east; intermittent hilly and mountainous regions in Great Plains, with occasional badland topography; rugged mountains and broad river valleys in Alaska; rugged, volcanic topography in Hawaii and the territories
Natural resources	coal, copper, lead, molybdenum, phosphates, rare earth elements, uranium, bauxite, gold, iron, mercury, nickel, potash, silver, tungsten, zinc, petroleum, natural gas, timber, arable land
Natural hazards	tsunamis; volcanoes; earthquake activity around Pacific Basin; hurricanes along the Atlantic and Gulf of Mexico coasts; tornadoes in the Midwest and Southeast; mud slides in California; forest fires in the west; flooding; permafrost in northern Alaska
Environmental issues	severe water shortages, air pollution resulting in acid rain in both the US and Canada
Exclusive economic zone	11,351,000 km2 (4,383,000 sq mi)

The term 'United States', when used in the geographical sense, refers to the contiguous United States, the state of Alaska, the island state of Hawaii, the five insular territories of Puerto Rico, Northern Mariana Islands, U.S. Virgin Islands, Guam, and American Samoa, and minor outlying possessions. The United States shares land borders with Canada and Mexico and maritime borders with Russia, Cuba, The Bahamas, and other countries, in addition to Canada and Mexico. The northern border of the United States with Canada is the world's longest bi-national land border.

Area

From 1989 through 1996, the total area of the US was listed as 9,372,610 km² (3,618,780 sq mi) (land + inland water only). The listed total area changed to 9,629,091 km² (3,717,813 sq mi) in 1997 (Great Lakes area and coastal waters added), to 9,631,418 km² (3,718,711 sq mi) in 2004, to 9,631,420 km² (3,718,710 sq mi) in 2006, and to 9,826,630 km² (3,794,080 sq mi) in 2007 (territorial waters added). Currently, the CIA World Factbook gives 9,826,675 km² (3,794,100 sq mi), the United Nations Statistics Division gives 9,629,091 km² (3,717,813 sq mi), and the Encyclopedia Britannica gives 9,522,055 km² (3,676,486 sq mi) (Great Lakes area included but not coastal waters). These sources consider only the 50 states and the Federal District and exclude overseas territories. The US has the 2nd largest Exclusive Economic Zone of 11,351,000 km² (4,383,000 sq mi).

By total area (water as well as land), the United States is either slightly larger or smaller than the People's Republic of China, making it the world's third or fourth-largest country. China and the United States are smaller than Russia and Canada in total area but are larger than Brazil. By land area only (exclusive of waters), the United States is the world's third largest country, after Russia and China, with Canada in fourth. Whether the US or China is the third largest country by total area depends on two factors: (1) The validity of China's claim on Aksai Chin and Trans-Karakoram Tract (both these territories are also claimed by India, so are not counted); and (2) How the US calculates its surface area. Since the initial publishing of the World Factbook, the CIA has updated the total area of the United States several times.

General characteristics

A satellite composite image of the contiguous United States. Temperate and Subtropical forests, of deciduous, coniferous, broadleaved evergreen, and mixed vegetation, as well as grasslands, montane forests, and croplands, prevail in the east, transitioning to prairies (semi-arid steppe), boreal forests, the Rockies in the west and deserts in the southwest. In the northeast, the coasts of the Great Lakes and Atlantic seaboard host much of the country's population.

 

A satellite composite image of the state of Hawaii. Volcanoes prevail on the Big Island. The islands have rugged coastlines, sandy beaches and a tropical environment, although temperatures and humidity tend to be less extreme because of near-constant trade winds from the east.

 

See also: Borders of the United States

The United States shares land borders with Canada (to the north) and Mexico (to the south), and a territorial water border with Russia in the northwest, and two territorial water borders in the southeast between Florida and Cuba, and Florida and the Bahamas. The contiguous forty-eight states are otherwise bounded by the Pacific Ocean on the west, the Atlantic Ocean on the east, and the Gulf of Mexico to the southeast. Alaska borders the Pacific Ocean to the south and southwest, the Bering Strait to the west, and the Arctic Ocean to the north, while Hawaii lies far to the southwest of the mainland in the Pacific Ocean.

Forty-eight of the states are in the single region between Canada and Mexico; this group is referred to, with varying precision and formality, as the contiguous United States, and as the Lower 48. Alaska, which is included in the term continental United States, is located at the northwestern end of North America.

The capital city, Washington, District of Columbia, is a federal district located on land donated by the state of Maryland. (Virginia had also donated land, but it was returned in 1849.) The United States also has overseas territories (Insular areas) with varying levels of autonomy and organization: in the Caribbean territories of Puerto Rico and the U.S. Virgin Islands (formerly the Danish Virgin Islands, purchased by the US at the beginning of WW2), and in the Pacific the inhabited territories of American Samoa, Guam and the Northern Mariana Islands, along with a number of uninhabited island territories. Some of the territories acquired were a part of United States imperialism, or to gain access to the east.

Nearly all of the United States is in the northern hemisphere—the exceptions are American Samoa and Jarvis Island.

Physiographic divisions

Main article: Physiographic regions of the United States

 

Denali, Alaska, is the highest point in North America at 20,310 ft (6,190.5 m).

The eastern United States has a varied topography. A broad, flat coastal plain lines the Atlantic and Gulf shores from the Texas-Mexico border to New York City, and includes the Florida peninsula. This broad coastal plain and barrier islands make up the widest and longest beaches in the United States, much of it composed of soft, white sands. The Florida Keys are a string of coral islands that reach the southernmost city on the United States mainland (Key West). Areas further inland feature rolling hills, mountains, a diverse collection of temperate and subtropical moist and wet forests. Parts of interior Florida and South Carolina are also home to sandhill communities. The Appalachian Mountains form a line of low mountains separating the eastern seaboard from the Great Lakes and the Mississippi Basin. New England features rocky seacoasts and rugged mountains with peaks up to 6200 feet and valleys dotted with rivers and streams. Offshore Islands dot the Atlantic and Gulf coasts. A recent global remote sensing analysis suggested that there were 6,622 km² of tidal flats in the United States, making it the 4th ranked country in terms of tidal flat area.

The five Great Lakes are located in the north-central portion of the country, four of them forming part of the border with Canada; only Lake Michigan is situated entirely within the United States. The southeast United States, generally stretching from the Ohio River southwards, includes a variety of warm temperate and subtropical moist and wet forests, as well as warm temperate and subtropical dry forests nearer the Great Plains in the west of the region. West of the Appalachians lies the lush Mississippi River basin and two large eastern tributaries, the Ohio River and the Tennessee River. The Ohio and Tennessee Valleys and the Midwest consist largely of rolling hills, interior highlands and small mountains, jungle-like marsh and swampland near the Ohio River, and productive farmland, stretching south to the Gulf Coast. The Midwest also has a vast amount of cave systems.

The Great Plains lie west of the Mississippi River and east of the Rocky Mountains. A large portion of the country's agricultural products are grown in the Great Plains. Before their general conversion to farmland, the Great Plains were noted for their extensive grasslands, from tallgrass prairie in the eastern plains to shortgrass steppe in the western High Plains. Elevation rises gradually from less than a few hundred feet near the Mississippi River to more than a mile high in the High Plains. The generally low relief of the plains is broken in several places, most notably in the Ozark and Ouachita Mountains, which form the U.S. Interior Highlands, the only major mountainous region between the Rocky Mountains and the Appalachian Mountains.

The Great Plains come to an abrupt end at the Rocky Mountains. The Rocky Mountains form a large portion of the Western U.S., entering from Canada and stretching nearly to Mexico. The Rocky Mountain region is the highest region of the United States by average elevation. The Rocky Mountains generally contain fairly mild slopes and wider peaks compared to some of the other great mountain ranges, with a few exceptions (such as the Teton Mountains in Wyoming and the Sawatch Range in Colorado). The highest peaks of the Rockies are found in Colorado, the tallest peak being Mount Elbert at 14,440 ft (4,400 m). In addition, instead of being one generally continuous and solid mountain range, it is broken up into a number of smaller, intermittent mountain ranges, forming a large series of basins and valleys.

West of the Rocky Mountains lies the Intermontane Plateaus (also known as the Intermountain West), a large, arid desert lying between the Rockies and the Cascades and Sierra Nevada ranges. The large southern portion, known as the Great Basin, consists of salt flats, drainage basins, and many small north–south mountain ranges. The Southwest is predominantly a low-lying desert region. A portion known as the Colorado Plateau, centered around the Four Corners region, is considered to have some of the most spectacular scenery in the world. It is accentuated in such national parks as Grand Canyon, Arches, Mesa Verde and Bryce Canyon, among others. Other smaller Intermontane areas include the Columbia Plateau covering eastern Washington, western Idaho and northeast Oregon and the Snake River Plain in Southern Idaho.

The Grand Canyon from Moran Point. The Grand Canyon is among the most famous locations in the country.

The Intermontane Plateaus come to an end at the Cascade Range and the Sierra Nevada. The Cascades consist of largely intermittent, volcanic mountains, many rising prominently from the surrounding landscape. The Sierra Nevada, further south, is a high, rugged, and dense mountain range. It contains the highest point in the contiguous 48 states, Mount Whitney (14,505 ft or 4,421 m). It is located at the boundary between California's Inyo and Tulare counties, just 84.6 mi or 136.2 km west-northwest of the lowest point in North America at the Badwater Basin in Death Valley National Park at 279 ft or 85 m below sea level.

These areas contain some spectacular scenery as well, as evidenced by such national parks as Yosemite and Mount Rainier. West of the Cascades and Sierra Nevada is a series of valleys, such as the Central Valley in California and the Willamette Valley in Oregon. Along the coast is a series of low mountain ranges known as the Pacific Coast Ranges.

Alaska contains some of the most dramatic scenery in the country. Tall, prominent mountain ranges rise up sharply from broad, flat tundra plains. On the islands off the south and southwest coast are many volcanoes. Hawaii, far to the south of Alaska in the Pacific Ocean, is a chain of tropical, volcanic islands, popular as a tourist destination for many from East Asia and the mainland United States.

The territories of Puerto Rico and the U.S. Virgin Islands encompass a number of tropical isles in the northeastern Caribbean Sea. In the Pacific Ocean the territories of Guam and the Northern Mariana Islands occupy the limestone and volcanic isles of the Mariana archipelago, and American Samoa (the only populated US territory in the southern hemisphere) encompasses volcanic peaks and coral atolls in the eastern part of the Samoan Islands chain.

Physiographic regions

A physiographical map of the contiguous 48 states of the U.S. The map indicates the age of the exposed surface as well as the type of terrain.

The geography of the United States varies across its immense area. Within the continental U.S., eight distinct physiographic divisions exist, though each is composed of several smaller physiographic subdivisions. These major divisions are:

• Laurentian Upland – part of the Canadian Shield that extends into the northern United States Great Lakes area.
• Atlantic Plain – the coastal regions of the eastern and southern parts include the continental shelf, the Atlantic Coast and the Gulf Coast.
• Appalachian Highlands – lying on the eastern side of the United States, it includes the Appalachian Mountains, the Watchung Mountains, the Adirondacks and New England province originally containing the Great Eastern Forest, a stretch of mixed temperature and subtropical montane forests, some of which are rainforests.
• Interior Plains – part of the interior continental United States, it includes the Great Plains, as well as a number of highland and mountainous regions, like the Black Hills, dense cave systems, painted hills and badland features.
• Interior Highlands – also part of the interior continental United States, this division includes the Ozark Plateau, the Ouachita Mountains, and other smaller mountain systems. This region is located largely in the warm temperate/subtropical moist and dry forest biomes.
• Rocky Mountain System – one branch of the Cordilleran system lying far inland in the western states.
• Intermontane Plateaus – also divided into the Columbia Plateau, the Colorado Plateau and the Basin and Range Province, it is a system of plateaus, basins, ranges and gorges between the Rocky and Pacific Mountain Systems. It is the setting for the Grand Canyon, the Great Basin and Death Valley.
• Pacific Mountain System – the coastal mountain ranges and features in the west coast of the United States.

Much of the central United States is covered by relatively flat, arable land. This aerial photo was taken over northern Ohio.

The Atlantic coast of the United States is low, with minor exceptions. The Appalachian Highland owes its oblique northeast–southwest trend to crustal deformations which in very early geological time gave a beginning to what later came to be the Appalachian Mountain system. This system had its climax of deformation so long ago (probably in Permian time) that it has since then been very generally reduced to moderate or low relief. It owes its present-day altitude either to renewed elevations along the earlier lines or to the survival of the most resistant rocks as residual mountains. The oblique trend of this coast would be even more pronounced but for a comparatively modern crustal movement, causing a depression in the northeast resulting in an encroachment of the sea upon the land. Additionally, the southeastern section has undergone an elevation resulting in the advance of the land upon the sea.

While the Atlantic coast is relatively low, the Pacific coast is, with few exceptions, hilly or mountainous. This coast has been defined chiefly by geologically recent crustal deformations, and hence still preserves a greater relief than that of the Atlantic. The low Atlantic coast and the hilly or mountainous Pacific coast foreshadow the leading features in the distribution of mountains within the United States.

The east coast Appalachian system, originally forest covered, is relatively low and narrow and is bordered on the southeast and south by an important coastal plain. The Cordilleran system on the western side of the continent is lofty, broad and complicated having two branches, the Rocky Mountain System and the Pacific Mountain System. In between these mountain systems lie the Intermontane Plateaus. Both the Columbia River and Colorado River rise far inland near the easternmost members of the Cordilleran system, and flow through plateaus and intermontane basins to the ocean. Heavy forests cover the northwest coast, but elsewhere trees are found only on the higher ranges below the Alpine region. The intermontane valleys, plateaus and basins range from treeless to desert with the most arid region being in the southwest.

The Laurentian Highlands, the Interior Plains and the Interior Highlands lie between the two coasts, stretching from the Gulf of Mexico northward, far beyond the national boundary, to the Arctic Ocean. The central plains are divided by a hardly perceptible height of land into a Canadian and a United States portion. It is from the United States side, that the great Mississippi system discharges southward to the Gulf of Mexico. The upper Mississippi and some of the Ohio basin is the semi-arid prairie region, with trees originally only along the watercourses. The uplands towards the Appalachians were included in the great eastern forested area, while the western part of the plains has an arid climate supporting only scanty native plant life, and in the south, it is practically barren.

Elevation extremes:

• Lowest point: Death Valley, Inyo County, California −280 ft (−85.3 m)
• Highest point: Denali, Denali Borough, Alaska 20,310 ft (6,190.5 m)

Climate

Main article: Climate of the United States

Köppen climate types of the US (including the 5 inhabited U.S. territories)

 

A map of average precipitation across the contiguous US.

Due to its large size and wide range of geographic features, the United States contains examples of nearly every global climate. The climate is subtropical in the Southern United States, continental in the north, tropical in Hawaii and southern Florida, polar in Alaska, semiarid in the Great Plains west of the 100th meridian, Mediterranean in coastal California and arid in the Great Basin and the Southwest. Its comparatively favorable agricultural climate contributed (in part) to the country's rise as a world power, with infrequent severe drought in the major agricultural regions, a general lack of widespread flooding, and a mainly temperate climate that receives adequate precipitation.

The main influence on U.S. weather is the polar jet stream which migrates northward into Canada in the summer months, and then southward into the US in the winter months. The jet stream brings in large low-pressure systems from the northern Pacific Ocean that enters the US mainland over the Pacific Northwest. The Cascade Range, Sierra Nevada, and Rocky Mountains pick up most of the moisture from these systems as they move eastward. Greatly diminished by the time they reach the High Plains, much of the moisture has been sapped by the orographic effect as it is forced over several mountain ranges.

Once it moves over the Great Plains, uninterrupted flat land allows it to reorganize and can lead to major clashes of air masses. In addition, moisture from the Gulf of Mexico is often drawn northward. When combined with a powerful jet stream, this can lead to violent thunderstorms, especially during spring and summer. Sometimes during winter, these storms can combine with another low-pressure system as they move up the East Coast and into the Atlantic Ocean, where they intensify rapidly. These storms are known as Nor'easters and often bring widespread, heavy rain, wind, and snowfall to New England. The uninterrupted grasslands of the Great Plains also lead to some of the most extreme climate swings in the world. Temperatures can rise or drop rapidly, winds can be extreme and the flow of heat waves or Arctic air masses often advance uninterrupted through the plains.

The Great Basin and Columbia Plateau (the Intermontane Plateaus) are arid or semiarid regions that lie in the rain shadow of the Cascades and Sierra Nevada. Precipitation averages less than 15 inches (38 cm). The Southwest is a hot desert, with temperatures exceeding 100 °F (37.8 °C) for several weeks at a time in summer. The Southwest and the Great Basin are also affected by the monsoon from the Gulf of California from July to September, which brings localized but often severe thunderstorms to the region.

Much of California consists of a Mediterranean climate, with sometimes excessive rainfall from October–April and nearly no rain the rest of the year. In the Pacific Northwest rain falls year-round but is much heavier during winter and spring. The mountains of the west receive abundant precipitation and very heavy snowfall. The Cascades are one of the snowiest places in the world, with some places averaging over 600 inches (1,524 cm) of snow annually, but the lower elevations closer to the coast receive very little snow.

Florida has a subtropical climate in the northern part of the state and a tropical climate in the southern part of the state. Summers are wet and winters are dry in Florida. Annually much of Florida, as well as the deep southern states, are frost-free. The mild winters of Florida allow a massive tropical fruit industry to thrive in the central part of the state, making the US second to only Brazil in citrus production in the world.

Another significant (but localized) weather effect is lake-effect snow that falls south and east of the Great Lakes, especially in the hilly portions of the Upper Peninsula of Michigan and on the Tug Hill Plateau in New York. The lake effect dumped well over 5 feet (1.52 m) of snow in the area of Buffalo, New York throughout the 2006–2007 winter. The Wasatch Front and Wasatch Range in Utah can also receive significant lake effect accumulations from the Great Salt Lake.

Extremes

See also: U.S. state temperature extremes

In northern Alaska, tundra and arctic conditions predominate, where the temperature has fallen as low as −80 °F (−62.2 °C). On the other end of the spectrum, Death Valley, California once reached 134 °F (56.7 °C), the highest temperature ever recorded on Earth.

On average, the mountains of the western states receive the highest levels of snowfall on Earth. The greatest annual snowfall level is at Mount Rainier in Washington, at 692 inches (1,758 cm); the record there was 1,122 inches (2,850 cm) in the winter of 1971–72. This record was broken by the Mt. Baker Ski Area in northwestern Washington which reported 1,140 inches (2,896 cm) of snowfall for the 1998–99 snowfall season. Other places with significant snowfall outside the Cascade Range are the Wasatch Mountains in Utah, the San Juan Mountains in Colorado, and the Sierra Nevada in California.

Pago Pago Harbor in American Samoa has the highest annual rainfall of any harbor in the world.

In the east, the region near the Great Lakes and the mountain of the Northeast receives the most snowfall, although such snowfall levels do not near snowfall levels in the western United States. Along the northwestern Pacific coast, rainfall is greater than anywhere else in the continental U.S., with Quinault Rainforest in Washington having an average of 137 inches (348 cm). Hawaii receives even more, with 404 inches (1,026 cm) measured annually in the Big Bog, in Maui. Pago Pago Harbor in American Samoa is the rainiest harbor in the world (because of the 523 meter Rainmaker Mountain). The Mojave Desert, in the southwest, is home to the driest locale in the U.S. Yuma, Arizona, has an average of 2.63 inches (6.7 cm) of precipitation each year.

In central portions of the U.S., tornadoes are more common than anywhere else on Earth and touch down most commonly in the spring and summer. Deadly and destructive hurricanes occur almost every year along the Atlantic seaboard and the Gulf of Mexico. The Appalachian region and the Midwest experience the worst floods, though virtually no area in the U.S. is immune to flooding. The Southwest has the worst droughts; one is thought to have lasted over 500 years and to have hurt Ancestral Pueblo peoples. The West is affected by large wildfires each year.

Natural disasters

See also: List of natural disasters in the United States

The United States is affected by a variety of natural disasters yearly. Although drought is rare, it has occasionally caused major disruption, such as during the Dust Bowl (1931–1942). Farmland failed throughout the Plains, entire regions were substantially depopulated, and dust storms ravaged the land.

A powerful tornado near Dimmitt, Texas on June 2, 1995.

Tornadoes and hurricanes

Main articles: Tornadoes in the United States and Hurricanes in the United States

The Great Plains and Midwest, due to the contrasting air masses, see frequent severe thunderstorms and tornado outbreaks during spring and summer with around 1,000 tornadoes occurring each year. The strip of land from north Texas north to Kansas and Nebraska and east into Tennessee is known as Tornado Alley, where many houses have tornado shelters and many towns have tornado sirens due to the very frequent tornado formations in the region.

Hurricanes are another natural disaster found in the US, which can hit anywhere along the Gulf Coast or the Atlantic Coast as well as Hawaii in the Pacific Ocean. Particularly at risk are the central and southern Texas coasts, the area from southeastern Louisiana east to the Florida Panhandle, peninsular Florida, and the Outer Banks of North Carolina, although any portion of the coast could be struck. The U.S. territories and possessions in the Caribbean, such as Puerto Rico and the U.S. Virgin Islands, are also vulnerable to hurricanes due to their location in the Caribbean Sea.

Hurricane season runs from June 1 to November 30, with a peak from mid-August through early October. Some of the more devastating hurricanes have included the Galveston Hurricane of 1900, Hurricane Andrew in 1992, Hurricane Katrina in 2005, and Hurricane Harvey and Hurricane Maria in 2017.

Hurricanes (known as cyclones in the Pacific Ocean) fail to landfall on the Pacific Coast of the United States due to water temperatures being too cool to sustain them. However, the remnants of tropical cyclones from the Eastern Pacific occasionally impact the western United States, bringing moderate to heavy rainfall.

Total devastation in Gulfport, Mississippi caused by storm surge from Hurricane Katrina in 2005.

Flooding

Occasional severe flooding is experienced in the United States. Significant floods throughout history include the Great Mississippi Flood of 1927, the Great Flood of 1993, and widespread flooding and mudslides caused by the 1982–83 El Niño event in the western United States. Flooding is still prevalent, mostly on the Eastern Coast, during hurricanes or other inclement weather, for example in 2012, when Hurricane Sandy devastated the region. Localized flooding can, however, occur anywhere, and mudslides from heavy rain can cause problems in any mountainous area, particularly the Southwest. Large stretches of desert shrub in the west can fuel the spread of wildfires. The narrow canyons of many mountain areas in the west and severe thunderstorm activity during the summer lead to sometimes devastating flash floods as well, while nor'easter snowstorms can bring activity to a halt throughout the Northeast (although heavy snowstorms can occur almost anywhere).

Geologic

The West Coast of the continental United States makes up part of the Pacific Ring of Fire, an area of heavy tectonic and volcanic activity that is the source of 90% of the world's earthquakes. The American Northwest sees the highest concentration of active volcanoes in the United States, in Washington, Oregon and northern California along the Cascade Mountains. There are several active volcanoes located in the islands of Hawaii, including Kilauea in ongoing eruption since 1983, but they do not typically adversely affect the inhabitants of the islands. There has not been a major life-threatening eruption on the Hawaiian Islands since the 17th century. Volcanic eruptions can occasionally be devastating, such as in the 1980 eruption of Mount St. Helens in Washington.

The Ring of Fire makes California and southern Alaska particularly vulnerable to earthquakes. Earthquakes can cause extensive damage, such as the 1906 San Francisco earthquake or the 1964 Good Friday earthquake near Anchorage, Alaska. California is well known for seismic activity and requires large structures to be earthquake resistant to minimize loss of life and property. Outside of devastating earthquakes, California experiences minor earthquakes on a regular basis.

There have been about 100 significant earthquakes annually from 2010 to 2012. Past averages were 21 a year. This is believed to be due to the deep disposal of wastewater from fracking. None has exceeded a magnitude of 5.6, and no one has been killed.

Other natural disasters

Other natural disasters include tsunamis around the Pacific Basin, mudslides in California, and forest fires in the western half of the contiguous U.S. Although drought is relatively rare, it has occasionally caused major economic and social disruption, such as during the Dust Bowl (1931–1942), which resulted in widespread crop failures and dust storms, beginning in the southern Great Plains and reaching to the Atlantic Ocean.

Consequences

According to report by U.S. Census Bureau, in 2022 natural disasters led to the forced displacement of 3.3 million people, more than 1.3% of the U.S. adult population, with half of the displacements being caused by the hurricanes. The survey-report stated that in Florida, the devastation caused by Hurricanes Ian and Nicole resulted in the relocation of around 1 million people, or about one in every 17 adult residents. In Louisiana, where inhabitants were still dealing with the devastating results of Hurricane Ida the year before, more than 409,000 people, or almost one in every eight residents, were moved. Despite this, the Louisiana state saw a relatively calm hurricane season in 2022.

Public lands

Exclusive economic zones of the United States, including insular areas

The United States holds many areas for the use and enjoyment of the public. These include national parks, national monuments, national forests, wilderness areas, and other areas. For lists of areas, see the following articles:

• List of National Parks of the United States
• List of National Natural Landmarks
• List of U.S. National Forests
• List of U.S. Wilderness Areas

Human

Main article: Demography of the United States

In terms of human geography, the United States is inhabited by a diverse set of ethnicities and cultures.