Anisotropic Carrier Transport Research Articles

Facet‐Dependent, Fast Response, and Broadband Photodetector Based on Highly Stable All‐Inorganic CsCu2I3 Single Crystal with 1D Electronic Structure

AbstractLow‐dimensional metal halides at molecular level, which feature strong quantum confinement effects from intrinsic structure, are emerging as ideal candidates in optoelectronic fields. However, developing stable and nontoxic metal halides still remains a great challenge. Herein, for the first time, high‐crystalline and highly stable CsCu2I3 single crystal, which is acquired by a low‐cost antisolvent vapor assisted method, is successfully developed to construct high‐speed (trise/tdecay = 0.19 ms/14.7 ms) and UV‐to‐visible broadband (300–700 nm) photodetector, outperforming most reported photodetectors based on individual all‐inorganic lead‐free metal halides. Intriguingly, facet‐dependent photoresponse is observed for CsCu2I3 single crystal, whose morphology consists of {010}, {110}, and {021} crystal planes. The on–off ratio of {010} crystal plane is higher than that of {110} crystal plane, mainly owing to lower dark current. Furthermore, photogenerated electrons are localized in twofold chains created by [CuI4] tetrahedra, leading to relatively small effective mass and fast transport mobility along the 1D transport pathway. Anisotropic carrier transport characteristic is related to stronger confinement and higher electron density for {110} crystal planes. This work not only demonstrates the great potential of CsCu2I3 single crystal in high‐performance optoelectronics, but also gives insights into 1D electronic structure associated with fast photoresponse and high anisotropy.

Vertical carrier transport in strain-balanced InAs/InAsSb type-II superlattice material

Anisotropic carrier transport properties of unintentionally doped InAs/InAs0.65Sb0.35 type-II strain-balanced superlattice material are evaluated using temperature- and field-dependent magnetotransport measurements performed in the vertical direction on a substrate-removed metal-semiconductor-metal device structure. To best isolate the measured transport to the superlattice, device fabrication entails flip-chip bonding and backside device processing to remove the substrate material and deposit contact metal directly to the bottom of an etched mesa. High-resolution mobility spectrum analysis is used to calculate the conductance contribution and corrected mixed vertical-lateral mobility of the two carrier species present. Combining the latter with lateral mobility results from in-plane magnetotransport measurements on identical superlattice material allows for the calculation of the true vertical majority electron and minority hole mobilities; amplitudes of 4.7 ×103 cm2/V s and 1.60 cm2/V s are determined at 77 K, respectively. The temperature-dependent results show that vertical hole mobility rapidly decreases with decreasing temperature due to trap-induced localization and then hopping transport, whereas vertical electron mobility appears phonon scattering-limited at high temperature, giving way to interface roughness scattering at low temperatures, analogous to the lateral electron mobility but with a lower overall magnitude.

Thickness effect on low-power driving of MoS2 transistors in balanced double-gate fields

In field-effect transistors (FETs), when the thickness of the semiconducting transition metal dichalcogenides (TMDs) channel exceeds the maximum depletion depth, the entire region cannot be completely controlled by a single-gate electric field. The layer-to-layer carrier transitions between the van der Waals interacted TMD layers result in the extraordinary anisotropic carrier transport in the in-plane and out-of-plane directions. The performance of the TMD FETs can be largely enhanced by optimizing the thickness of the TMD channel as well as increasing the effective channel area through which the gate field is delivered. In this study, we investigated the carrier behavior and device performance in double-gate FETs fabricated using a 57 nm thick MoS2, which is thicker than the maximum depletion depth of about 50 nm, and a much thinner 4 nm thick MoS2. The results showed that in the thick MoS2, the gate voltages at both ends formed two independent channels which had no synergistic effect on the device performance owing to the inefficient delivery of the vertical electric field. On the other hand, in the thin MoS2 channel, the double-gate voltages effectively controlled one channel, resulting in twice the carrier mobility and operation in a low electric field region, i.e. below 0.2 MV cm−1.

Electronic properties and polaronic dynamics of semi-Dirac system within Ladder approximation

We investigate the electronic properties of the semi-Dirac system and its polaronic dynamics when coupled with a Fermi bath with quadratic dispersion. The electronic anisotropic transport properties and the semiclassical dynamics of the semi-Dirac system are studied, including the density-of-states, conductivity, transport relaxation rate, specific heat, electrical current density, and free energy. The attractive polaron formed as the semi-Dirac impurity dressed with the particle–hole excitations in a two-dimensional system are studied both analytically and numerically. The pair propagator, self-energy, spectral function are being detailly calculated and discussed. The method of medium T-matrix approximation (nonself-consistent), which equivalent to the partially dressed interaction vertex by summing over all ladder diagrams, is applied, and compared with some other methods (for many-body problem), like the leading-order 1/N expansion (GW approximation), Hartree–Fock theory, and the Nozieres–Schmitt–Rink theory. Since we focus on the weak-coupling region, the mean-field approximation is also applicable. The polaron properties is related to the anisotropic effective masses of the semi-Dirac system. That’s in contrast to the polarons formed in the surface of normal Dirac systems which has an isotropic dispersion, since the anisotropic dispersion of the semi-Dirac systems results in anisotropic effective mass and anisotropic charge carrier transport. Besides, the symmetry between electron and hole is also broken since the effective masses of the electron and hole are different, which are affected by the polaronic effect when the semi-Dirac material is deposited on a polar substrate, like hBN. The self-localization, short-range potential, and experimental methods as well as the possible formation of the Bose polaron on the surface of semi-Dirac system are also discussed in the end. Our results are useful also for the investigation of two/three-dimensional bosonic polaron as well as the polarons in other solid state systems, like the magnetic matter or the topological systems.

Filiform Metal Silver Nanoinclusions To Enhance Thermoelectric Performance of P-type Ca3Co4O9+δ Oxide.

Cd doping and metallic Ag additives in Ca3Co4O9+δ polycrystalline materials are shown to result in improved thermoelectric (TE) transport properties. Carrier concentration and mobility were optimized through the combination of doping and compositional modulation approaches. The formation of filiform Ag nanoinclusions between the interlayers and grain boundaries enhances the anisotropic carrier transport, leading to higher carrier mobility. A spin entropy enhancement due to the change of the net valence of Co induced by Cd substitution on the Ca site was confirmed by X-ray photoelectron spectroscopy. High carrier mobility and enhanced spin entropy results in higher electrical conductivity and Seebeck coefficient, leading to the increase of the power factor. In conjunction, mass fluctuation between Cd and Ca on the same crystal site along with the increase of metallic Ag nanoinclusions effectively lowers thermal conductivity. Consequently, the figure-of-merit, zT, has been improved to 0.31 at 950 K for 10 wt % Ag-modified Ca2.9Cd0.1Co4O9+δ specimen, which is a significant improvement compared to the pristine material. This dual-mode control of electron and phonon transport by including Ag additives and Cd doping offers an approach for tuning the correlated TE parameters.

Optoelectrical anisotropy in graphene oxide supported polythiophene thin films fabricated by floating film transfer

A facile method for the fabrication of highly oriented hybrid thin films of conjugated polymer (CP) with graphene oxide (GO) as 2D nano-filler over the high surface tension liquid substrate by floating film transfer is being reported. Formation of the self-assembled oriented hybrid thin film has been validated using multiple characterization techniques. Hybrid thin films thus prepared have been used for the fabrication of organic field effect transistors (OFETs) followed by investigation of anisotropic charge carrier transport. OFETs made with hybrid films oriented in channel direction exhibited the superior device performance with >6 fold augmentation in field effect mobility (μ) and one order improvement in on/off ratio as compared to that of devices based on pristine CP. However, pristine polymer shows 2.2 fold enhancement in mobility and one order augmentation in on/off ratio in parallel orientation as compared to the corresponding perpendicularly oriented films. Thus, our study has elucidated a path to obtain oriented, aligned, and self - assembled hybrid films for further improvement in the device performance aiming towards the applications in organic electronics.

Complementary Black Phosphorus Nanoribbons Field-Effect Transistors and Circuits

This paper demonstrates a high-performance black phosphorus nanoribbons field-effect transistor (BPNR-FET) and systematically investigates methods for enhancing its anisotropic carrier transport. The BPNR-FET shows a strong dependence on crystal orientation in which the best mobility performance is achieved in armchair-oriented nanoribbons. A downscaling of nanoribbon width is shown to improve the short-channel effect owing to a better electrostatic gate control. Furthermore, hydrogenation is employed to effectively passivate the dangling bonds and heal the nanoribbon edge defects, leading to nearly hysteresis-free transfer properties. By virtue of bandgap and contact-metal workfunction engineering, n-type BPNR-FET is successfully demonstrated, which enables complementary inverter circuits to be simultaneously realized. This paper unravels the superior performance underscores a conceptually new BPNR-FET, paving the way toward the development of non-planar devices and integrated circuits based on 2-D materials platform.

Anisotropic Carrier Transport in CH3NH3PbI3 Single Crystal Field-Effect Transistor

By constructing vertical-structured field-effect transistors with CH3NH3PbI3 single-crystalline films of different orientations, anisotropic carrier transport in CH3NH3PbI3 single crystal was observed. Compared with the [100] and [112] orientation, carriers show lower mobility along the [001] orientation. The anisotropy of carrier transport is attributed to the migration of $\mathrm {CH}_{3}\mathrm {NH}_{3}^{+}$ under electric field.

Carrier transport in layered nanolaminated carbides

This paper summarizes the ab-initio electronic and phonon band structures and the temperature-dependent carrier transport in layered Ti2AlC. It is found that the cylindrical Fermi surface is the origin of the anisotropic carrier effective mass (infinite effective mass along the c axis), which leads to a strong anisotropic (insulator along the c axis and metallic along the layer) carrier transport in these films. Using electronic and phonon band structure calculations, we have developed an analytical model for the carrier-phonon interaction and the in-plane carrier conductivity originating from the strong inter-valley (s → d) scattering in Ti2AlC. The density functional theory is used to calculate the average deformation potential corresponding to the acoustic phonon vibrations. The calculated deformation potential is in good agreement with the extracted deformation potential from the transport data available in the literature. The extracted deformation potential will be useful for predicting the transport quantities of these metals at elevated temperatures.

Spatial-Temporal Imaging of Anisotropic Photocarrier Dynamics in Black Phosphorus.

As an emerging single elemental layered material with a low symmetry in-plane crystal lattice, black phosphorus (BP) has attracted significant research interest owing to its unique electronic and optoelectronic properties, including its widely tunable bandgap, polarization-dependent photoresponse and highly anisotropic in-plane charge transport. Despite extensive study of the steady-state charge transport in BP, there has not been direct characterization and visualization of the hot carriers dynamics in BP immediately after photoexcitation, which is crucial to understanding the performance of BP-based optoelectronic devices. Here we use the newly developed scanning ultrafast electron microscopy (SUEM) to directly visualize the motion of photoexcited hot carriers on the surface of BP in both space and time. We observe highly anisotropic in-plane diffusion of hot holes with a 15 times higher diffusivity along the armchair (x-) direction than that along the zigzag (y-) direction. Our results provide direct evidence of anisotropic hot carrier transport in BP and demonstrate the capability of SUEM to resolve ultrafast hot carrier dynamics in layered two-dimensional materials.

Temperature-dependent anisotropic charge-carrier mobility limited by ionized impurity scattering in thin-layer black phosphorus

Anisotropic charge carrier transport in black phosphorus limited by ionized impurity scattering at finite temperature is explored theoretically. The anisotropic electronic structure enters the calculation for the polarizability (screening), the momentum relaxation time, and the mobility. For finite temperature, scattering is not limited to the Fermi surface and the polarizability is temperature dependent. The impact of screening is investigated in detail with its dependence on carrier density and temperature. Competing with the thermal excitation effects, the temperature dependence of the polarizability is found to dominate for T<100K. As a result, the charge carrier mobility slowly decreases with increasing temperature. The weak temperature dependence of the mobility and its anisotropy ratio of 1.9-3.2 agree with published experimental data.

Anisotropic charge carrier transport in free-standing hexagonal boron nitride thin films

The in-plane and out-of-plane mobility–lifetime products of electrons and holes in free-standing hexagonal boron nitride (hBN) films are extracted from current–voltage characteristics of metal–hBN–metal structures measured under external excitations. The in-plane mobility–lifetime products for electrons and holes are ∼2.8 × 10−5 and ∼4.85 × 10−6 cm2/V, measured from lateral carrier collection, whereas the out-of-plane mobility–lifetime products for electrons and holes are ∼5.8 × 10−8 and ∼6.1 × 10−9 cm2/V, measured from vertical carrier collection, respectively. The mobility–lifetime product is a few orders of magnitude higher along the plane than along the out of plane in hBN films.

Carrier Transport Mechanism in Single Crystalline Organic Semiconductor Thin Film Elucidated by Visualized Carrier Motion.

Time-resolved microscopic second harmonic generation (TRM-SHG) measurement was conducted to evaluate temperature dependence of the anisotropic carrier transport process in 6,13-Bis(triisopropylsilylethynyl) (TIPS) pentacene single crystalline domains for two orthogonal directions. Enhancement of the electric field induced SHG (EFI-SHG) signal at the electrode edge at low temperature suggests the presence of potential drop in the injection process. We directly evaluated temperature dependence of the carrier mobility by taking into account the potential drop, and concluded that the Marcus theory is appropriate to interpret the carrier transport in anisotropic TIPS pentacene thin film. TRM-SHG method is a facile and effective way to directly visualize transport process in anisotropic materials and to evaluate injection and transport processes simultaneously.

Imaging Luminescent Traps on Single Anatase TiO2 Crystals: The Influence of Surface Capping on Photoluminescence and Charge Transport

The spatial distribution of intra band gap traps in micrometer-sized single crystals of anatase TiO2 was explored using single-particle photoluminescence (PL) spectroscopy and imaging. The PL from microcrystals with well-defined {001} and {101} facets was imaged for the same particle before and after annealing to explore the influence of fluorine, used as a capping agent in the synthesis of the microparticles, on luminescent traps. Unannealed particles reveal weaker photoluminescence and distinctly different spatial distribution of PL emission compared to annealed particles. The results show that the capped particles have fewer surface defects as a result of passivation of surface electron traps associated with undercoordinated titanium. PL images suggest that the remaining surface defects in the fluorine-capped particles concentrate at edges and corners of the microcrystal, and that anisotropic carrier transport takes place via hopping between adjacent Ti atoms. Annealed particles reveal greater surface ...

Experimental Studies on the Anisotropic Thermoelectric Properties of Conducting Polymer Films.

We reported general methods for studying the thermoelectric properties of a polymer film in both the in-plane and through-plane directions. The bench-mark PEDOT/PSS films have highly anisotropic carrier transport properties and thermal conductivity. The anisotropic carrier transport properties can be explained by the lamellar structure of the PEDOT/PSS films where the PEDOT nanocrystals could be isolated by the insulating PSS in the through-plane direction. The anisotropic thermal conductivity was mainly attributed to the lattice contribution from PSS because the polymer chain is oriented along the substrate.

Chemically synthesized heterostructures of two-dimensional molybdenum/tungsten-based dichalcogenides with vertically aligned layers.

We demonstrate a one-step, facile chemical synthesis of heterostructures based on two-dimensional (2D) transition metal dichalcogenides (TMDCs) with vertically aligned 2D layers. Specifically, by using a chemical vapor transport method, we synthesize molybdenum (Mo)/tungsten (W)-based disulfide and diselenide heterostructures, such as MoS2/WS2 and MoSe2/WSe2. Detailed structural characterizations reveal that the 2D layers of two different TMDCs are vertically standing/aligned exposing the edge sites of the layers. The TMDC heterostructure materials exhibit a high uniformity of structural/chemical heterogeneity over a large area (>1 cm(2)) and show anisotropic carrier transport properties. These materials with unique heterostructures have great implications for both technological applications and fundamental sciences.

Investigation of carrier transport properties in semipolar (112¯2) GaN films with low defect density

We report on the anisotropic carrier transport properties of semipolar (112¯2) GaN films with low defect density. We utilized the asymmetric lateral epitaxy to obtain various semipolar (112¯2) GaN films having significantly reduced partial dislocations and basal-plane stacking faults (BPSFs). The directionally dependent carrier transport was observed with the lower sheet resistances (Rsh) along the [11¯00] direction. The Rsh ratios of semipolar (112¯2) GaN films were found to be relatively smaller than those of nonpolar a-plane GaN films, possibly due to low BPSF density and the reduced in-plane electric field induced by BPSF along the [112¯3] direction at wurtzite domain boundaries.

Diameter-dependent photocatalytic performance of niobium pentoxide nanowires

Thermal reactions of a niobium oxo-oxalate complex, (NH4)3[NbO(Ox)3]·H2O, with ternary amines having more than two methylene chain units as structure directing solvents led to self-assemblies suitable for one dimensional growth to afford amorphous niobium oxide nanowires whose diameters depended on methylene chain lengths in the amines. These nanowires were crystallized at 773 K to obtain the TT phase of Nb2O5 whilst maintaining the same structure as before calcination. There was almost no difference in crystallinities, BET surface areas, and growth directions among the Nb2O5 nanowires. PtCl6(2-) was reduced by photogenerated electrons in the conduction band of the Nb2O5 nanowires following UV light irradiation (λ > 300 nm) in the presence of an electron donor. Metallic Pt nanoparticles were selectively deposited on a short axis plane of the nanowires prepared with tridodecylamine, whereas the others had the Pt nanoparticles throughout the surfaces. This indicated that photogenerated electrons moved along the nanowire growth direction associated with transfer of holes to the nanowire sidewall. The anisotropic carrier transport afforded the highest activity for photocatalytic H2 evolution reactions under UV light irradiation (λ > 300 nm).

Comprehensive Approach to Intrinsic Charge Carrier Mobility in Conjugated Organic Molecules, Macromolecules, and Supramolecular Architectures

Si-based inorganic electronics have long dominated the semiconductor industry. However, in recent years conjugated polymers have attracted increasing attention because such systems are flexible and offer the potential for low-cost, large-area production via roll-to-roll processing. The state-of-the-art organic conjugated molecular crystals can exhibit charge carrier mobilities (μ) that nearly match or even exceed that of amorphous silicon (1-10 cm(2) V(-1) s(-1)). The mean free path of the charge carriers estimated from these mobilities corresponds to the typical intersite (intermolecular) hopping distances in conjugated organic materials, which strongly suggests that the conduction model for the electronic band structure only applies to μ > 1 cm(2) V(-1) s(-1) for the translational motion of the charge carriers. However, to analyze the transport mechanism in organic electronics, researchers conventionally use a disorder formalism, where μ is usually less than 1 cm(2) V(-1) s(-1) and dominated by impurities, disorders, or defects that disturb the long-range translational motion. In this Account, we discuss the relationship between the alternating-current and direct-current mobilities of charge carriers, using time-resolved microwave conductivity (TRMC) and other techniques including field-effect transistor, time-of-flight, and space-charge limited current. TRMC measures the nanometer-scale mobility of charge carriers under an oscillating microwave electric field with no contact between the semiconductors and the metals. This separation allows us to evaluate the intrinsic charge carrier mobility with minimal trapping effects. We review a wide variety of organic electronics in terms of their charge carrier mobilities, and we describe recent studies of macromolecules, molecular crystals, and supramolecular architecture. For example, a rigid poly(phenylene-co-ethynylene) included in permethylated cyclodextrin shows a high intramolecular hole mobility of 0.5 cm(2) V(-1) s(-1), based on a combination of flash-photolysis TRMC and transient absorption spectroscopy (TAS) measurements. Single-crystal rubrene showed an ambipolarity with anisotropic charge carrier transport along each crystal axis on the nanometer scale. Finally, we describe the charge carrier mobility of a self-assembled nanotube consisting of a large π-plane of hexabenzocoronene (HBC) partially appended with an electron acceptor. The local (intratubular) charge carrier mobility reached 3 cm(2) V(-1) s(-1) for the nanotubes that possessed well-ordered π-stacking, but it dropped to 0.7 cm(2) V(-1) s(-1) in regions that contained greater amounts of the electron acceptor because those molecules reduced the structural integrity of π-stacked HBC arrays. Interestingly, the long-range (intertubular) charge carrier mobility was on the order of 10(-4) cm(2) V(-1) s(-1) and monotonically decreased when the acceptor content was increased. These results suggest the importance of investigating charge carrier mobilities by frequency-dependent charge carrier motion for the development of more efficient organic electronic devices.

Molecular orientation and anisotropic carrier mobility in poorly soluble polythiophene thin films

Oriented thin films of a poorly soluble polythiophene, poly[benzo[1,2-d:4,5-d′]bisthiazole-2,6-diyl(3′,4,4″,4″′-tetradodecyl[2,2′:5′,2″:5″,2″′-quaterthiophene]-5,5″′-diyl)], were fabricated using the friction-transfer technique. The detail of the film structure was investigated using polarized UV-vis spectroscopy, transmission electron microscopy, and grazing-incidence x-ray diffraction observations. The friction-transfer technique enables control of the anisotropic carrier transport according to the face-on and/or edge-on molecular orientations in films with one-directionally aligned polymer backbones.