RESUMEN

The defects have a remarkable influence on the electronic structures and the electric transport behaviors of the matter, providing the additional means to engineering their physical properties. In this work, a comprehensive study on the effect of Br-vacancies on the electronic structures and transport behaviors in the high-order topological insulator Bi4Br4 is performed by the combined techniques of the scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES), and physical properties measurement system along with the first-principle calculations. The STM results show the defects on the cleaved surface of a single crystal and reveal that the defects are correlated to the Br-vacancies with the support of the simulated STM images. The role of the Br-vacancies in the modulation of the band structures has been identified by ARPES spectra and the calculated energy-momentum dispersion. The relationship between the Br-vacancies and the semiconducting-like transport behaviors at low temperature has been established, implying a Mott variable ranging hopping conduction in Bi4Br4. The work not only resolves the unclear transport behaviors in this matter, but also paves a way to modulate the electric conduction path by the defects engineering.

RESUMEN

The confinement of electrons in one-dimensional (1D) space highlights the prominence of the role of electron interactions or correlations, leading to a variety of fascinating physical phenomena. The quasi-1D electron states can exhibit a unique spin texture under spin-orbit interaction (SOI) and thus could generate a robust spin current by forbidden electron backscattering. Direct detection of such 1D spin or SOI information, however, is challenging due to complicated techniques. Here, we identify an anomalous planar Hall effect (APHE) in the magnetotransport of quasi-1D van der Waals (vdW) topological materials as exemplified by Bi4Br4, which arises from the quantum interference correction of 1D weak antilocalization (WAL) to the ordinary planar Hall effect and demonstrates a deviation from the usual sine and cosine curves. The occurrence of 1D WAL is correlated to the line-shape Fermi surface and persistent spin texture of (100) topological surface states of Bi4Br4, as revealed by both our angle-resolved photoemission spectroscopy and first-principles calculations. By generalizing the observation of APHE to other non-vdW bulk materials, this work provides a possible characteristic of magnetotransport for identifying the spin/SOI information and quantum interference behavior of 1D states in 3D topological material.

RESUMEN

We study the effect of a local potential shift induced by a side electrode on the edge modes at the boundary between gapped and ungapped bilayer graphene. A potential shift close to the gapped-ungapped boundary causes the emergence of unprotected edge modes, propagating in both directions along the boundary. These counterpropagating edge modes allow edge backscattering, as opposed to the case of valley-momentum-locked edge modes. We then calculate the conductance of a bilayer graphene wire in presence of finger-gate electrodes, finding strong asymmetries with energy inversion and deviations from conductance quantization that can be understood with the gate-induced unprotected edge modes.

RESUMEN

Classical nanofluidic frameworks account for the confined fluid and ion transport under an electrostatic field at the solid-liquid interface, but the electronic property of the solid is often overlooked. Harvesting the interaction of the nanofluidic transport with the electron transport in solid requires a route effectively coupling ion and electron dynamics. Here we report a nanofluidic analogy of Coulomb drag for exploring the dynamic ion-electron interactions at the liquid-graphene interface. An induced electric current in graphene by ionic flow with no bias directly applied to the graphene channel is observed experimentally, featuring an opposite electron current direction to the ion current. Our experiments and ab initio calculations show that the current generation stems from the confined ion-electron interactions via a nanofluidic Coulomb drag mechanism. Our findings may open up a new dimension for nanofluidics and transport control by ion-electron coupling.

RESUMEN

Transition metal dichalcogenides exhibit rich phase diagrams dominated by the interplay of superconductivity and charge density waves, which often result in anomalies in the electric transport properties. Here, we employ the ionic gating technique to realize a tunable, non-volatile organic ion intercalation in bulk single crystals of molybdenum disulphide (MoS2). We demonstrate that this gate-driven organic ion intercalation induces a strong electron doping in the system without changing the pristine 2H crystal symmetry and triggers the emergence of a re-entrant insulator-to-metal transition. We show that the gate-induced metallic state exhibits clear anomalies in the temperature dependence of the resistivity with a natural explanation as signatures of the development of a charge-density wave phase which was previously observed in alkali-intercalated MoS2. The relatively large temperature at which the anomalies are observed (∼150 K), combined with the absence of any sign of doping-induced superconductivity down to ∼3 K, suggests that the two phases might be competing with each other to determine the electronic ground state of electron-doped MoS2.

RESUMEN

The surface functionalization of two-dimensional (2D) materials with organic electron donors (OEDs) is a powerful tool to modulate the electronic properties of the material. Here we report a novel molecular dopant, Me-OED, that demonstrates record-breaking molecular doping to MoS2, achieving a carrier density of 1.10 ± 0.37 × 1014 cm-2 at optimal functionalization conditions; the achieved carrier density is much higher than those by other OEDs such as benzyl viologen and an OED based on 4,4'-bipyridine. This impressive doping power is attributed to the compact size of Me-OED, which leads to high surface coverage on MoS2. To confirm, we study tBu-OED, which has an identical reduction potential to Me-OED but is significantly larger. Using field-effect transistor measurements and spectroscopic characterization, we estimate the doping powers of Me- and tBu-OED are 0.22-0.44 and 0.11 electrons per molecule, respectively, in good agreement with calculations. Our results demonstrate that the small size of Me-OED is critical to maximizing the surface coverage and molecular interactions with MoS2, enabling us to achieve unprecedented doping of MoS2.

RESUMEN

Highly ordered nanostructure arrays have attracted wide attention due to their wide range of applicability, particularly in fabricating devices containing scalable and controllable junctions. In this work, highly ordered carbon nanotube (CNT) arrays grown directly on Si substrates were fabricated, and their electronic transport properties as a function of wall thickness were explored. The CNTs were synthesized by chemical vapor deposition inside porous alumina membranes, previously fabricated on n-type Si substrates. The morphology of the CNTs, controlled by the synthesis parameters, was characterized by electron microscopies and Raman spectroscopy, revealing that CNTs exhibit low crystallinity (LC). A study of conductance as a function of temperature indicated that the dominant electric transport mechanism is the 3D variable range hopping. The electrical transport explored by I-V curves was approached by an equivalent circuit based on a Schottky diode and resistances related to the morphology of the nanotubes. These junction arrays can be applied in several fields, particularly in this work we explored their performance in gas sensing mode and found a fast and reliable resistive response at room temperature in devices containing LC-CNTs with wall thickness between 0.4 nm and 1.1 nm.

RESUMEN

The electric transport properties of flexible and transparent conducting bilayers, realized by sputtering ultrathin gold nanometric layers on sodium-alginate free-standing films, were studied. The reported results cover a range of temperatures from 3 to 300 K. In the case of gold layer thicknesses larger than 5 nm, a typical metallic behavior was observed. Conversely, for a gold thickness of 4.5 nm, an unusual resistance temperature dependence was found. The dominant transport mechanism below 70 K was identified as a fluctuation-induced tunneling process. This indicates that the conductive region is not continuous but is formed by gold clusters embedded in the polymeric matrix. Above 70 K, instead, the data can be interpreted using a phenomenological model, which assumes an anomalous expansion of the conductive region upon decreasing the temperature, in the range from 300 to 200 K. The approach herein adopted, complemented with other characterizations, can provide useful information for the development of innovative and green optoelectronics.

RESUMEN

Low-symmetry two-dimensional (2D) semiconductors have attracted great attention because of their rich in-plane anisotropic optical, electrical, and thermoelectric properties and potential applications in multifunctional nanoelectronic and optoelectronic devices. However, anisotropic 2D semiconductors with high performance are still very limited. Here, we report the systematic study of in-plane anisotropic properties in few-layered b-As that is a narrow-gap semiconductor, based on the experimental and theoretical investigations. According to experimental results, we have come up with a simple method for identifying the orientation of b-As crystals. Meanwhile, we show that the maximum mobility of electrons and holes was measured in the in-plane armchair (AC) direction. The measured maximum electron mobility ratio is about 2.68, and the hole mobility ratio is about 1.79.

RESUMEN

We report a facile approach based on piezoelectric-driven nanotips inside a scanning electron microscope to contact and electrically characterize ultrathin MoS2 (molybdenum disulfide) flakes on a SiO2/Si (silicon dioxide/silicon) substrate. We apply such a method to analyze the electric transport and field emission properties of chemical vapor deposition-synthesized monolayer MoS2, used as the channel of back-gate field effect transistors. We study the effects of the gate-voltage range and sweeping time on the channel current and on its hysteretic behavior. We observe that the conduction of the MoS2 channel is affected by trap states. Moreover, we report a gate-controlled field emission current from the edge part of the MoS2 flake, evidencing a field enhancement factor of approximately 200 and a turn-on field of approximately   40   V / µ m at a cathode-anode separation distance of 900   nm .

RESUMEN

Although carbon nanotube transistors present outstanding performances based on key metrics, large-scale uniformity and repeatability required in printable electronics depend greatly on proper control of the electrostatic environment. Through a survey of polymer dielectric encapsulants compatible with printing processes, a simple correlation is found between the measured interfacial charge density and the onset of conduction in a transistor, providing a rational route to control the electrical characteristics of carbon nanotube transistors. Smooth and continuous balancing of the properties between unipolar p-type and n-type transport is achieved using a molar fraction series of poly(styrene-co-2-vinylpyridine) statistical copolymers combined with an electron-donating molecule. We further demonstrate the easy fabrication of a p-n diode which shows a modest rectification of 8:1.

RESUMEN

The interface engineering of two-dimensional (2D) transition-metal dichalcogenides (TMDs) has been regarded as a promising strategy to modulate their outstanding electrical and optoelectronic properties because of their inherent 2D nature and large surface-to-volume ratio. In particular, introducing organic molecules and polymers directly onto the surface of TMDs has been explored to passivate the surface defects or achieve better interfacial properties with neighboring surfaces efficiently, thus leading to great opportunities for the realization of high-performance TMD-based applications. This review provides recent progress in the interface engineering of TMDs with organic molecules and polymers corresponding to the modulation of their electrical and optoelectronic characteristics. Depending on the interfaces between the surface of TMDs and dielectric, conductive contacts or the ambient environment, we present various strategies to introduce an organic interlayer from materials to processing. In addition, the role of native defects on the surface of TMDs, such as adatoms or vacancies, in determining their electrical characteristics is also discussed in detail. Finally, the future challenges and opportunities associated with the interface engineering are highlighted.

RESUMEN

We present a method for a bottom-up synthesis of atomically thin graphene sheets with tunable crystallinity and porosity using aromatic self-assembled monolayers (SAMs) as molecular precursors. To this end, we employ SAMs with pyridine and pyrrole constituents on polycrystalline copper foils and convert them initially into molecular nanosheets-carbon nanomembranes (CNMs)- via low-energy electron irradiation induced cross-linking and then into graphene monolayers via pyrolysis. As the nitrogen atoms are leaving the nanosheets during pyrolysis, nanopores are generated in the formed single-layer graphene. We elucidate the structural changes upon the cross-linking and pyrolysis down to the atomic scale by complementary spectroscopy and microscopy techniques including X-ray photoelectron and Raman spectroscopy, low energy electron diffraction, atomic force, helium ion, and high-resolution transmission electron microscopy, and electrical transport measurements. We demonstrate that the crystallinity and porosity of the formed graphene can be adjusted via the choice of molecular precursors and pyrolysis temperature, and we present a kinetic growth model quantitatively describing the conversion of molecular CNMs into graphene. The synthesized nanoporous graphene monolayers resemble a percolated network of graphene nanoribbons with a high charge carrier mobility (∼600 cm2/(V s)), making them attractive for implementations in electronic field-effect devices.

RESUMEN

PbS nanocrystals are surface-functionalized with the organic semiconductor 5,5″-dithiol-[2,2':5,2″-terthiophene] and assembled to afford hybrid nanostructured thin films with a large structural coherence and an electron mobility of 0.2 cm2/(V s). Electrochemistry, optical spectroscopy, and quantum mechanical calculations are applied to elucidate the electronic structure at the inorganic/organic interface, and it is established that electron injection into the molecule alters its (electronic) structure, which greatly facilitates coupling of the neighboring PbS 1Se states. This is verified by field-effect and electrochemically gated transport measurements, and evidence is provided that carrier transport occurs predominantly via the 1Se states. The presented material allows studying structure-transport correlations and exploring transport anisotropies in semiconductor nanocrystal superlattices.

RESUMEN

Nanoporous graphene- based electric-double-layer transistors (EDLTs) are successfully fabricated. Transport measurements of the EDLTs demonstrate that the ambipolar electronic states of massless Dirac fermions with a high carrier mobility are well preserved in 3D nanoporous graphene along with anomalous nonlinear Hall resistance and exceptional transistor on/off ratio. This study may open a new avenue for device applications of graphene.

RESUMEN

In the past, nanoporous metal-organic frameworks (MOFs) have been mostly studied for their huge potential with regard to gas storage and separation. More recently, the discovery that the electrical conductivity of a widely studied, highly insulating MOF, HKUST-1, improves dramatically when loaded with guest molecules has triggered a huge interest in the charge carrier transport properties of MOFs. The observed high conductivity, however, is difficult to reconcile with conventional transport mechanisms: neither simple hopping nor band transport models are consistent with the available experimental data. Here, we combine theoretical results and new experimental data to demonstrate that the observed conductivity can be explained by an extended hopping transport model including virtual hops through localized MOF states or molecular superexchange. Predictions of this model agree well with precise conductivity measurements, where experimental artifacts and the influence of defects are largely avoided by using well-defined samples and the Hg-drop junction approach.

RESUMEN

We present a method to create and erase spatially resolved doping profiles in graphene-hexagonal boron nitride heterostructures. The technique is based on photoinduced doping by a focused laser beam and does neither require masks nor photoresists. This makes our technique interesting for rapid prototyping of unconventional electronic device schemes, where the spatial resolution of the rewritable, long-term stable doping profiles is limited by only the laser spot size (≈600 nm) and the accuracy of sample positioning. Our optical doping method offers a way to implement and to test different, complex doping patterns in one and the very same graphene device, which is not achievable with conventional gating techniques.

RESUMEN

We report measured quantized conductance and quasi-ballistic transport in selectively regrown In0.85Ga0.15As nanowires. Very low parasitic resistances obtained by regrowth techniques allow us to probe the near-intrinsic electrical properties, and we observe several quantized conductance steps at 10 K. We extract a mean free path of 180 ± 40 nm and an effective electron mobility of 3300 ± 300 cm(2)/V·s, both at room temperature, which are among the largest reported values for nanowires of similar dimensions. In addition, optical characterization of the nanowires by photoluminescence and Raman measurement is performed. We find an unintentional increase of indium in the InxGa1-xAs composition relative to the regrown film layer, as well as partial strain relaxation.

RESUMEN

Understanding of the electric transport through surface-anchored metal-organic frameworks (SURMOFs) is important both from a fundamental perspective as well as with regards to possible future applications in electronic devices. To address this mostly unexplored subject, we integrated a series of representative SURMOF thin films, formed by copper nodes and trimesic acid and known as HKUST-1, in a mercury-drop-based tunneling junction. Although the transport properties of these SURMOFs are analogous to those of hybrid metal-organic molecular wires, manifested by a very low value of the tunneling decay constant (ß ≈ 0.006 Å(-1)), they are at the same time found to be consistent with a linear increase of resistance with film thickness. Upon loading of SURMOF pores with ferrocene (Fc), a noticeable increase in transport current was observed. A transport model and ab initio electronic structure calculations were used to reveal a hopping transport mechanism and to relate the changes upon Fc loading to those of the electronic and vibrational structures of the SURMOF films.

RESUMEN

Se presentan las propiedades eléctricas del compuesto Cu3BiS3 depositado por co-evaporación. Este es un nuevo compuesto que puede tener propiedades adecuadas para ser utilizado como capa absorbente en celdas solares. Las muestras fueron caracterizadas a través de medidas de efecto Hall y fotovoltaje superficial transiente (SPV). A través de medidas de efecto Hall se encontró que la concentración de portadores de carga n es del orden de 10(16) cm-3 independiente de la relación de masas de Cu/Bi. También se encontró que la movilidad de este compuesto (μ del orden de 4 cm²V -1s-1) varía de acuerdo con los mecanismos de transporte que la gobiernan en dependencia con la temperatura. A partir de las medidas de SPV se encontró alta densidad de defectos superficiales, defectos que son pasivados al superponer una capa buffer sobre el compuesto Cu3BiS3.

Here, we present the electrical properties of the compound Cu3BiS3 deposited by co-evaporation. This new compound may have the properties necessary to be used as an absorbent layer in solar cells. The samples were characterized by Hall effect and transient surface photovoltage (SPV) measurements. Using Hall effect measurements, we found that the concentration of n charge carriers is in the order of 10(16) cm³ irrespective of the Cu/Bi mass ratio. We also found that the mobility of this compound (μ in the order of 4 cm² V-1s-1) varies according to the transport mechanisms that govern it and are dependent on temperature. Based on the SPV, we found a high density of surface defects, which can be passivated by superimposing a buffer layer over the Cu3BiS3 compound.

Apresentam-se as propriedades elétricas do composto Cu3BiS3 depositado por co-evaporação. É um composto novo que pode ter as propriedades adequadas para ser utilizado como capa absorvente em células solares. As amostras foram caracterizadas através de medidas do efeito Hall e foto voltagem superficial transiente (SPV). Através de medidas do efeito Hall se encontro que a concentração de portadores de carga n é da ordem de 10(16) cm³ independentemente da relação de massas de Cu/Bi. Também se encontrou que a mobilidade des composto (μ da ordem de 4 cm²V -1s-1) varia de acordo com os mecanismos de transporte que a governam em dependência com a temperatura. Partindo das medidas de SPV se encontrou uma alta densidade de defeitos superficiais, defeitos que são passivados a sobrepor uma capa buffer sobre o composto Cu3BiS3.