Search

Indium selenide (In2Se3) has got much recent attraction in a variety of research areas mainly due to its multiphase structure. Many existing studies, however, have focused on growth of α and β phase, while chemical vapor deposition (CVD) growth of the γ-phase has only been achieved for micon-thick flakes with the assistance of iodine trichloride (ICl3) catalysts. Here, we present the “catalyst-free” CVD growth of β and γ phase In2Se3 flakes with thicknesses down to only a few nanometers. The structural, optical and ferroelectric properties of the grown samples were investigated carefully by using various techniques. Optical microscope and Raman spectroscopy studies revealed that triangular and hexagonal shaped β and γ phases were grown at different substrate temperatures, respectively. The second-harmonic generation (SHG) measurement confirmed that β has centrosymmetric structure and γ phase has non-centrosymmetric structure. The ferroelectric properties of the γ-In2Se3 were measured along in-plane (IP) and out-of-plane (OOP) directions, for the first time, by employing piezo-force microscope (PFM). The non-monotonic thickness dependence of the ferroelectricity was found and explained with the combination of Raman and SHG measurements. The catalyst-free growth of γ-In2Se3 and the observation of ferroelectricity in it would be a valuable addition in the field of ferroelectric and piezoelectric switching devices.

Thickness matters in graphene stacks If you stack graphene monolayers on top of each other, the number of layers will affect the properties of the material. Intuitively, one would expect that as the stack becomes thicker, the results will converge as the sample starts to resemble graphite. Nam et al. measured the conductance of graphene multilayers of increasing thickness. Studying samples up to seven layers thick, they found that in all of them, electronic correlations caused a phase transition at a nonzero critical temperature. However, the critical temperature, as well as the nature of the low-temperature state, depended strongly on the number of layers. This unexpectedly persistent dependence showed no signs of slowing down and will motivate further theoretical and experimental work. Science , this issue p. 324

Film thickness dependent changes in the topological properties of magnetic solitons are tracked via magnetotransport measurements in exfoliated crystals of the monoaxial chiral helimagnet Cr${}_{1/3}$NbS${}_{2}$.

Ballistic transport occurs whenever electrons propagate without collisions deflecting their trajectory. It is normally observed in conductors with a negligible concentration of impurities, at low temperature, to avoid electron-phonon scattering. Here, we use suspended bilayer graphene devices to reveal a new regime, in which ballistic transport is not limited by scattering with phonons or impurities, but by electron-hole collisions. The phenomenon manifests itself in a negative four-terminal resistance that becomes visible when the density of holes (electrons) is suppressed by gate-shifting the Fermi level in the conduction (valence) band, above the thermal energy. For smaller densities transport is diffusive, and the measured conductivity is reproduced quantitatively, with no fitting parameters, by including electron-hole scattering as the only process causing velocity relaxation. Experiments on a trilayer device show that the phenomenon is robust and that transport at charge neutrality is governed by the same physics. Our results provide a textbook illustration of a transport regime that had not been observed previously and clarify the nature of conduction through charge-neutral graphene under conditions in which carrier density inhomogeneity is immaterial. They also demonstrate that transport can be limited by a fully electronic mechanism, originating from the same microscopic processes that govern the physics of Dirac-like plasmas., Submitted

As competing with the established silicon technology, organic-inorganic metal halide perovskites are continually gaining ground in optoelectronics due to their excellent material properties and low-cost production. The ability to have control over their shape, as well as composition and crystallinity, is indispensable for practical materialization. Many sophisticated nanofabrication methods have been devised to shape perovskites; however, they are still limited to in-plane, low-aspect-ratio, and simple forms. This is in stark contrast with the demands of modern optoelectronics with freeform circuitry and high integration density. Here, a nanoprecision 3D printing is developed for organic-inorganic metal halide perovskites. The method is based on guiding evaporation-induced perovskite crystallization in mid-air using a femtoliter ink meniscus formed on a nanopipette, resulting in freestanding 3D perovskite nanostructures with a preferred crystal orientation. Stretching the ink meniscus with a pulling process enables on-demand control of the nanostructure's diameter and hollowness, leading to an unprecedented tubular-solid transition. With varying the pulling direction, a layer-by-layer stacking of perovskite nanostructures is successfully demonstrated with programmed shapes and positions, a primary step for additive manufacturing. It is expected that the method has the potential to create freeform perovskite nanostructures for customized optoelectronics.

Close to charge neutrality, the low-energy properties of high-quality suspended devices based on atomically thin graphene layers are determined by electron-electron interactions. Bernal-stacked layers, in particular, have shown a remarkable even-odd effect with mono- and tri-layers remaining gapless conductors, and bi- and tetra-layers becoming gapped insulators. These observations $-$at odds with the established notion that (Bernal) trilayers and thicker multilayers are semi-metals$-$ have resulted in the proposal of a physical scenario leading to a surprising prediction, namely that even-layered graphene multilayers remain insulating irrespective of their thickness. Here, we present data from two devices that conform ideally to this hypothesis, exhibiting the behavior expected for Bernal-stacked hexa and octalayer graphene. Despite their large thickness, these multilayers are insulating for carrier density |n, 22 pages, 6 figures, To appear in 2D Materials

The absence of an energy gap separating valence and conduction bands makes the low-energy electronic properties of graphene and its multi-layers sensitive to electron-electron interactions. In bilayers, for instance, interactions are predicted to open a gap at charge neutrality, turning the system into an insulator, as observed experimentally. In mono and (Bernal-stacked) trilayers, interactions, although still important, do not have an equally drastic effect, and these systems remain conducting at low temperature. It may be expected that interaction effects become weaker for thicker multilayers, whose behavior should eventually converge to that of graphite. Here we show that this expectation does not correspond to reality by investigating the case of Bernal-stacked tetralayer graphene (4LG). We reveal the occurrence of a robust insulating state in a narrow range of carrier densities around charge neutrality, incompatible with the behavior expected from the single-particle band structure. The phenomenology resembles that observed in bilayers, but the stronger conductance suppression makes the insulating state in 4LG visible at higher temperature. To account for our findings, we suggest a natural generalization of the interaction-driven, symmetry-broken states proposed for bilayers. This generalization also explains the systematic even-odd effect of interactions in Bernal-stacked layers of different thickness that is emerging from experiments, and has implications for the multilayer-to-graphite crossover., 15 pages, 5 figures, This is the original submitted version of the manuscript whose final accepted version, following the review/editorial process, will appear in Nature Communications

Interfacial interactions allow the electronic properties of graphene to be modified, as recently demonstrated by the appearance of satellite Dirac cones in graphene on hexagonal boron nitride substrates. Ongoing research strives to explore interfacial interactions with other materials to engineer targeted electronic properties. Here we show that with a tungsten disulfide (WS2) substrate, the strength of the spin–orbit interaction (SOI) in graphene is very strongly enhanced. The induced SOI leads to a pronounced low-temperature weak anti-localization effect and to a spin-relaxation time two to three orders of magnitude smaller than in graphene on conventional substrates. To interpret our findings we have performed first-principle electronic structure calculations, which confirm that carriers in graphene on WS2 experience a strong SOI and allow us to extract a spin-dependent low-energy effective Hamiltonian. Our analysis shows that the use of WS2 substrates opens a possible new route to access topological states of matter in graphene-based systems., Routes towards inducing strong spin–orbit coupling in graphene have been hindered by detrimental effects on its electronic properties and material quality. Here, the authors demonstrate a possible solution by exploiting interfacial interactions between graphene and a tungsten disulfide substrate.

We have performed systematic investigations of transport through graphene on hexagonal boron nitride (hBN) substrates, together with confocal Raman measurements and a targeted theoretical analysis, to identify the dominant source of disorder in this system. Low-temperature transport measurements on many devices reveal a clear correlation between the carrier mobility $\mu$ and the width $n^*$ of the resistance peak around charge neutrality, demonstrating that charge scattering and density inhomogeneities originate from the same microscopic mechanism. The study of weak-localization unambiguously shows that this mechanism is associated to a long-ranged disorder potential, and provides clear indications that random pseudo-magnetic fields due to strain are the dominant scattering source. Spatially resolved Raman spectroscopy measurements confirm the role of local strain fluctuations, since the line-width of the Raman 2D-peak --containing information of local strain fluctuations present in graphene-- correlates with the value of maximum observed mobility. The importance of strain is corroborated by a theoretical analysis of the relation between $\mu$ and $n^*$ that shows how local strain fluctuations reproduce the experimental data at a quantitative level, with $n^*$ being determined by the scalar deformation potential and $\mu$ by the random pseudo-magnetic field (consistently with the conclusion drawn from the analysis of weak-localization). Throughout our study, we compare the behavior of devices on hBN substrates to that of devices on SiO$_2$ and SrTiO$_3$, and find that all conclusions drawn for the case of hBN are compatible with the observations made on these other materials. These observations suggest that random strain fluctuations are the dominant source of disorder for high-quality graphene on many different substrates, and not only on hexagonal boron nitride., Comment: 14 pages, 6 figures, To appear in Physical Review X

We introduce a new scheme to realize suspended, multi-terminal graphene structures that can be current annealed successfully to obtain uniform, very high quality devices. A key aspect is that the bulky metallic contacts are not connected directly to the part of graphene probed by transport measurements, but only through etched constriction, which prevents the contacts from acting invasively. The device high quality and uniformity is demonstrated by a reproducibly narrow (delta_n ~ 10^9 cm^-2) resistance peak around charge neutrality, by carrier mobility values exceeding 10^6 cm^2/V/s, by the observation of integer quantum Hall plateaus starting at 30 mT and of symmetry broken states at about 200 mT, and by the occurrence of a negative multi-terminal resistance directly proving the occurrence of ballistic transport. As these multi-terminal devices enable measurements that cannot be done in a simpler two-terminal configuration, we anticipate that their use in future studies of graphene-based systems will be particularly relevant., Comment: 22 pages, 7 figures, including supporting information

We investigate low-temperature magneto-transport in recently developed, high-quality multi-terminal suspended bilayer graphene devices, enabling the independent measurement of the longitudinal and transverse resistance. We observe clear signatures of the fractional quantum Hall effect, with different states that are either fully developed, and exhibit a clear plateau in the transverse resistance with a concomitant dip in longitudinal resistance, or incipient, and exhibit only a longitudinal resistance minimum. All observed states scale as a function of filling factor nu, as expected. An unprecedented even-denominator fractional state is observed at nu = -1/2 on the hole side, exhibiting a clear plateau in Rxy quantized at the expected value of 2h/e^2 with a precision of ~0.5%. Many of our observations, together with a recent electronic compressibility measurement performed in graphene bilayers on hexagonal boron-nitride (hBN) substrates, are consistent with a recent theory that accounts for the effect of the degeneracy between the N=0 and N=1 Landau levels in the fractional quantum Hall effect, and predicts the occurrence of a Moore-Read type nu = -1/2 state. Owing to the experimental flexibility of bilayer graphene --which has a gate-dependent band structure, can be easily accessed by scanning probes, and can be contacted with materials such as superconductors--, our findings offer new possibilities to explore the microscopic nature of even-denominator fractional quantum Hall effect., 16 pages, 4 figures, appear in Nano Letters (Article ASAP)

We have developed a process to fabricate suspended graphene devices with local bottom gates, and tested it by realizing electrostatically controlled pn junctions on a suspended graphene mono-layer nearly 2 micrometers long. Measurements as a function of gate voltage, magnetic field, bias, and temperature exhibit characteristic Fabry-Perot oscillations in the cavities formed by the pn junction and each of the contacts, with transport occurring in the ballistic regime. Our results demonstrate the possibility to achieve a high degree of control on the local electronic properties of ultra-clean suspended graphene layers, a key aspect for the realization of new graphene nanostructures., Comment: 10 pages, 4 figures

To minimize parasitic doping effects caused by uncontrolled material adsorption, graphene is often investigated under vacuum. Here we report an entirely unexpected phenomenon occurring in vacuum systems, namely strong n-doping of graphene due to chemical species generated by common ion high-vacuum gauges. The effect --reversible upon exposing graphene to air-- is significant, as doping rates can largely exceed 10^{12} cm^{-2}/hour, depending on pressure and the relative position of the gauge and the graphene device. It is important to be aware of the phenomenon, as its basic manifestation can be mistakenly interpreted as vacuum-induced desorption of p-dopants., 10 pages, 4 figures

Suspended graphene nano-ribbons formed during current annealing of suspended graphene flakes have been investigated experimentally. Transport measurements show the opening of a transport gap around charge neutrality due to the formation of "Coulomb islands", coexisting with quantum Hall conductance plateaus appearing at moderate values of magnetic field $B$. Upon increasing $B$, the transport gap is rapidly suppressed, and is taken over by a much larger energy gap due to electronic correlations. Our observations show that suspended nano-ribbons allow the investigation of phenomena that could not so far be accessed in ribbons on SiO$_2$ substrates., 5 pages and 5 figures, Accepted in Physical Review Letters

We fabricated graphene pnp devices, by embedding pre-defined local gates in an oxidized surface layer of a silicon substrate. With neither dielectric-material deposition nor electron-beam irradiation on the graphene, we obtained high-quality graphene pnp devices without degradation of the carrier mobility even in the local-gate region. The corresponding increased mean free path leads to the observation of ballistic and phase-coherent transport across a 130-nm-wide local gate, which is about an order of magnitude wider than reported previously. Furthermore, in our scheme, we demonstrated independent control of the carrier density in the local-gate region, with a conductance map very distinctive from top-gated devices. It was caused as the electric field arising from the global back gate is strongly screened by the embedded local gate. Our scheme allows the realization of ideal multipolar graphene junctions with ballistic carrier transport., accepted in Nanotechnology

A spin injection is achieved in a direct-contact cobalt$\ensuremath{-}$single-layer graphene nonlocal spin-valve system, overlaid with a top gate. The spin signal is retained even in bipolar configurations of graphene. Hanle spin-precession analysis demonstrates that proportionality between spin and momentum relaxation times, which supports the Elliot-Yafet-type spin relaxation, holds consistently only when the carrier-density dependence of the density of states is taken into account. The corresponding strong spin-orbit coupling ($\ensuremath{\sim}$10 meV) suggests that covalently bonded adsorbates, rather than charged impurities, govern the spin relaxation in diffusive graphene.

Thermoelectric power (TEP) is measured in bilayer graphene for various temperatures and charge-carrier densities. At low temperatures, measured TEP well follows the semiclassical Mott formula with a hyperbolic dispersion relation. TEP for a high carrier density shows a linear temperature dependence, which demonstrates a weak electron-phonon interaction in the bilayer graphene. For a low carrier density, a deviation from the Mott relation is observed at high temperatures and is attributed to the low Fermi temperature in the bilayer graphene. Oscillating TEP and the Nernst effect for varying carrier density, observed in a high magnetic field, are qualitatively explained by the two dimensionality of the system., Comment: published version

By using four-terminal configurations, we investigated the dependence of longitudinal and diagonal resistances of a graphene p-n interface on the quantum-Hall edge-state equilibration position. The resistance of a p-n device in our four-terminal scheme is asymmetric with respect to the zero point where the filling factor ($\nu$) of the entire graphene vanishes. This resistance asymmetry is caused by the chiral-direction-dependent change of the equilibration position and leads to a deeper insight into the equilibration process of the quantum-Hall edge states in a bipolar graphene system., Comment: 5 pages, 4 figures, will be published in PRB

In this study, we determined the chiral direction of the quantum-Hall (QH) edge states in graphene by adopting simple two-terminal conductance measurements while grounding different edge positions of the sample. The edge state with a smaller filling factor is found to more strongly interact with the electric contacts. This simple method can be conveniently used to investigate the chirality of the QH edge state with zero filling factor in graphene, which is important to understand the symmetry breaking sequence in high magnetic fields ($\gtrsim$25 T)., Comment: 3 pages, 3 figures. Appeared in APL

Four-terminal resistances, both longitudinal and diagonal, of a locally gated graphene device are measured in the quantum-Hall (QH) regime. In sharp distinction from previous two-terminal studies [J. R. Williams \textit{et al.}, Science {\bf 317}, 638 (2007); B. \"{O}zyilmaz \textit{et al.}, Phys. Rev. Lett. {\bf 99}, 166804 (2007)], asymmetric QH resistances are observed, which provide information on reflection as well as transmission of the QH edge states. Most quantized values of resistances are well analyzed by the assumption that all edge states are equally populated. Contrary to the expectation, however, a 5/2 transmission of the edge states is also found, which may be caused by incomplete mode mixing and/or by the presence of counter-propagating edge states. This four-terminal scheme can be conveniently used to study the edge-state equilibration in locally gated graphene devices as well as mono- and multi-layer graphene hybrid structures., Comment: 6 pages, 4 figures. Typos and equations (2-4) have been corrected. Phys. Rev. B 79, 195327 (2009)

Charge carriers in a graphene sheet, a single layer of graphite, exhibit much distinctive characteristics to those in other two-dimensional electronic systems because of their chiral nature. In this report, we focus on the observation of weak localization in a graphene sheet exfoliated from a piece of natural graphite and nano-patterned into a Hall-bar geometry. Much stronger chiral-symmetry-breaking elastic intervalley scattering in our graphene sheet restores the conventional weak localization. The resulting carrier-density and temperature dependence of the phase coherence length reveal that the electron-electron interaction including a direct Coulomb interaction is the main inelastic scattering factor while electron-hole puddles enhance the inelastic scattering near the Dirac point., 12 pages, 3 figures, submitted to PRB

We report the observation of an unexpected, long-range field-effect in WTe$_2$ devices, leading to large gate-induced changes of the transport properties of crystals much thicker than the electrostatic screening length. The phenomenon --which manifests itself very differently from the conventional field-effect-- originates from the non-local nature of transport in the devices that are thinner than the carrier mean free path, because of the dominant role of surface scattering. We reproduce theoretically the gate dependence of the measured classical and quantum magneto-transport in all detail, and show that the phenomenon is caused by the gate-tuning of the bulk carrier mobility by changing the scattering at the surface. Our results demonstrate the possibility to gate tune the electronic properties deep in the interior of conducting materials, avoiding limitations imposed by electrostatic screening., Comment: To appear in Physical Review Letters soon (Published online October 19th, 2016), 10 pages, 8 figures, including the Supplemental Material