Single-crystal Flakes Research Articles

Quantum Hall effect of Dirac surface states of as-grown single crystal flakes in Sn0.02-Bi1.08Sb0.9Te2S without gate control

The quantum spin Hall effect (QSHE) in three-dimensional topological insulators (3D-TIs) is an important hallmark of realization of ultralow dissipative electrical transport originating from nontrivial topological surface Dirac states (TSDSs). The observation of QSHE, however, had been limited at low temperatures for thin film crystals prepared by epitaxial growth or ultrathin (nanometer-level) flakes, and the finite Fermi energy tuning was required using field effect transistor (FET) construction. Here, we show that a clear observation of QSHE with a quantized level of e2/h can be possible for high quality single crystal flakes of Sn-(Bi,Sb)2(Te,S)3 (Sn-BSTS) prepared by an exfoliation method without any gate control under FET construction. The QSHE is observed in an extremely large sample area of millimeter square at the largest thickness of micrometer and up to the highest temperature of 20 K, which is greatly superior to those reported for other 3D-TIs so far. The accurate band picture of both top and bottom topological surface states of Sn-BSTS is determined from both the QSHE and the Shubnikov-de Hass (SdH) oscillations. High quality Sn-BSTS of 3D-TIs will effectively be used for experiments targeting on science as well as many technological applications expected in the future.

Thin-Film Deposition of Surface Passivated Black Phosphorus.

A single-step, direct silicon-substrate growth of black phosphorus (BP) crystals is achieved in a self-contained short-way transport technique under low-pressure conditions (<1.5 MPa). A 115 nm-thick BP hero single crystal is formed with lateral dimensions of 10 × 85 μm. The synthesis proceeds with Sn, SnI4, and red phosphorus and has a well-defined phosphorus phase dependency on the SnI4 concentration. Furthermore, in situ Sn passivation of BP occurs. This allows long-term stability with no sign of any degradation after 4 months of exposure to ambient conditions. Single-crystal BP flakes and multigrain flakes with high- and low-angle grain boundaries are achieved. Electron backscatter diffraction determined crystal growth to be independent of the substrate, which is further supported by successful growth on various substrates, including sapphire, silicon nitride, silicon, and silicon oxide. Cross-sectional transmission electron microscopy of a single crystal flake provides valuable insight into the growth mechanism. Elemental Sn encapsulates BP crystals, and crystalline SnI x inclusions are found to be scattered throughout the BP crystal. It is suggested that SnI x inclusions may provide the dominant mechanism for seeding vertical growth. IR absorption measurements for thin and bulk BP recipes show an equal response below Eg dominated by free carrier absorption. FET devices fabricated from thin-film and bulk BP recipes show improved device performance compared to unpassivated BP films of equal thickness with an on/off current ratio >102.

Two-dimensional hybrid perovskites sustaining strong polariton interactions at room temperature.

Polaritonic devices exploit the coherent coupling between excitonic and photonic degrees of freedom to perform highly nonlinear operations with low input powers. Most of the current results exploit excitons in epitaxially grown quantum wells and require low-temperature operation, while viable alternatives have yet to be found at room temperature. We show that large single-crystal flakes of two-dimensional layered perovskite are able to sustain strong polariton nonlinearities at room temperature without the need to be embedded in an optical cavity formed by highly reflecting mirrors. In particular, exciton-exciton interaction energies are shown to be spin dependent, remarkably similar to the ones known for inorganic quantum wells at cryogenic temperatures, and more than one order of magnitude larger than alternative room temperature polariton devices reported so far. Because of their easy fabrication, large dipolar oscillator strengths, and strong nonlinearities, these materials pave the way for realization of polariton devices at room temperature.

Experimental setup for anisotropic thermoelectric transport measurements using MPMS

At reduced dimensions the electrical transport properties of materials often depend on the measurement direction. Here we report on a measurement setup designed on the manual insertion utility probe (MIUP) of the Quantum Design’s magnetic property measurement system (MPMS) to measure anisotropic transport properties in the temperature range of 10–400 K and magnetic fields up to 5 Tesla. The setup is capable of measuring the Seebeck coefficient and electrical resistivity both along and perpendicular to the applied magnetic field. The Seebeck measurement is based on the differential measurement technique; the four-probe electrical resistivity measurement can easily be performed on the opposite side of the setup. The setup consists of a small copper cube (5 × 5 × 5 mm3) fitted with diagonally cut electrically insulated square strips. The strips are fitted with copper-constantan thermocouple in differential arrangement for the temperature-gradient (ΔT) measurement and symmetrically arranged surface mount (SM) resistors to create the ΔT across the sample. The cube can be rotated to alter the direction of the applied magnetic field on the sample. We demonstrate the directional dependence of the measured transport properties for a normal platinum wire and for single crystal flakes of CuCr2Se4.

Intrinsic Carrier Transport of Phase-Pure Homologous 2D Organolead Halide Hybrid Perovskite Single Crystals.

This work reveals the intrinsic carrier transport behavior of 2D organolead halide perovskites based on phase-pure homologous (n = 1, 2, and 3) Ruddelsden-Popper perovskite (RPP) (BA)2 (MA)n -1 Pbn I3n+1 single crystals. The 2D perovskite field effect transistors with high-quality exfoliated 2D perovskite bulk crystals are fabricated, and characteristic output and transfer curves are measured from individual single-crystal flakes with various n values under different temperatures. Unipolar n-type transport dominated the electrical properties of all these 2D RPP single crystals. The transport behavior of the 2D organolead halide hybrid perovskites exhibits a strong dependence on the n value and the mobility substantially increases as the ratio of the number of inorganic perovskite slabs per organic spacer increases. By extracting the effect of contact resistances, the corrected mobility values for n = 1, 2, and 3 are 2 × 10-3 , 8.3 × 10-2 , and 1.25 cm2 V-1 s-1 at 77 K, respectively. Furthermore, by combining temperature-dependent electrical transport and optical measurements, it is found that the origin of the carrier mobility dependence on the phase transition for 2D organolead halide perovskites is very different from that of their 3D counterparts. Our findings offer insight into fundamental carrier transport behavior of 2D organic-inorganic hybrid perovskites based on phase-pure homologous single crystals.

Preparation and characterization of free-standing BiI3 single-crystal flakes for X-ray detection application

Free-standing BiI3 single-crystal flakes with a rhombohedral appearance and 10–100 µm in thickness were prepared by using physical vapor transport method. The (00l) (l = 3, 6, 9, 12) planes of the as-grown crystal present a distribution of layered structure and approximatively atomically smooth surface with 1.3–1.7 nm step height and roughness average 0.38–0.65 nm. Both planar and coplanar electrode configuration devices for X-ray detection were fabricated with the as-grown BiI3 single crystals and the dark resistivity 5.8–6.4 × 1011 Ω cm and 1.2–1.8 × 1011 Ω cm at room temperature were obtained, respectively. A low dark current or high resistivity for planar device is on account of carrier scattering from the I–Bi–I interlayer van der Waals bonding interface. A high net photocurrent and good sensitivity 1.22–1.36 × 104 µC/Gyair/cm2 under X-ray excitation were also obtained with planar device owing to the uniform electric field distribution and high charge collection efficiency.

Tunable Out-of-Plane Excitons in 2D Single-Crystal Perovskites

Hybrid organic-inorganic perovskites have emerged as very promising materials for photonic applications, thanks to the great synthetic versatility that allows to tune their optical properties. In the two-dimensional (2D) crystalline form, these materials behave as multiple quantum-well heterostructures with stable excitonic resonances up to room temperature. In this work strong light-matter coupling in 2D perovskite single-crystal flakes is observed, and the polarization-dependent exciton-polariton response is used to disclose new excitonic features. For the first time, an out-of-plane component of the excitons is observed, unexpected for such 2D systems and completely absent in other layered materials, such as transition-metal dichalcogenides. By comparing different hybrid perovskites with the same inorganic layer but different organic interlayers, it is shown how the nature of the organic ligands controllably affects the out-of-plane exciton-photon coupling. Such vertical dipole coupling is particularly sought in those systems, e.g. plasmonic nanocavities, in which the direction of the field is usually orthogonal to the material sheet. Organic interlayers are shown to affect also the strong birefringence associated to the layered structure, which is exploited in this work to completely rotate the linear polarization degree in only few microns of propagation, akin to what happens in metamaterials.

Magnetic vortex state in a layered muscovite sheet silicate single crystal

Natural muscovite, also called white-mica—a member in the sheet silicate mineral family, has been successfully exfoliated down to few mono-layers by mechanical exfoliation technique. The exfoliated muscovite single crystal flakes have been successfully transferred onto various substrates. Atomic force microscopy study was performed on exfoliated flakes transferred onto glass substrates; the thinnest flake measured had a thickness of ∼4.9 nm. Analysis made on the muscovite crystal structure and cleavage property clearly proved that the muscovite is another kind of layered 2D-material similar to graphene, MoS2, NiPS3 etc. Infrared absorption, diffuse reflectance and Raman spectroscopy studies were carried out. Muscovite exhibited an optical energy wide-band gap of ∼4.65 eV and exhibited a high intense Raman mode at 3620 cm−1, which we report here, for the first time. As the muscovite contains Fe, it exhibited interesting anisotropic magnetization properties. The field dependent magnetization showed signatures which well resembles that of an in-plane magnetic vortex state, normal and twisted onion states of magnetization configurations.

Single-Crystal Antimonene Films Prepared by Molecular Beam Epitaxy: Selective Growth and Contact Resistance Reduction of the 2D Material Heterostructure.

Single-crystal antimonene flakes are observed on sapphire substrates after the postgrowth annealing procedure of amorphous antimony (Sb) droplets prepared by using molecular beam epitaxy at room temperature. The large wetting angles of the antimonene flakes to the sapphire substrate suggest that an alternate substrate should be adopted to obtain a continuous antimonene film. By using a bilayer MoS2/sapphire sample as the new substrate, a continuous and single-crystal antimonene film is obtained at a low growth temperature of 200 °C. The results are consistent with the theoretical prediction of the lower interface energy between antimonene and MoS2. The different interface energies of antimonene between sapphire and MoS2 surfaces lead to the selective growth of antimonene only atop MoS2 surfaces on a prepatterned MoS2/sapphire substrate. With similar sheet resistance to graphene, it is possible to use antimonene as the contact metal of 2D material devices. Compared with Au/Ti electrodes, a specific contact resistance reduction up to 3 orders of magnitude is observed by using the multilayer antimonene as the contact metal to MoS2. The lower contact resistance, the lower growth temperature, and the preferential growth to other 2D materials have made antimonene a promising candidate as the contact metal for 2D material devices.

(Invited) Epitaxy of 2D Transition Metal Dichalcogenide Monolayers and Heterostructures

There is growing interest in monolayer transition metal dichalcogenides (TMDs) such as MoS2 and WSe2 and related 2D heterostructures due to their compelling electronic and photonic properties. The majority of this work has been carried out using ultra-thin flakes exfoliated from bulk chalcogenide crystals but future device development requires the ability to synthesize large area single crystal films and heterostructures. Our research is aimed at the development of an epitaxial growth technology for layered dichalcogenides, similar to that which exists for III-V and other compound semiconductors, based on gas source chemical vapor deposition (CVD). This approach provides a high overpressure of chalcogen species needed to maintain stable growth at elevated temperature and excellent control of the precursor partial pressures to achieve layer-by-layer and heterostructure growth. Our recent studies have focused on the epitaxial growth of WSe2 and WS2 monolayer films and heterostructures using metal hexacarbonyl and hydride chalcogen precursors on substrates including sapphire and hexagonal boron nitride (hBN). A multi-step precursor modulation growth method was developed to independently control nucleation density and the lateral growth rate of WSe2 and WS2 monolayer domains on the substrate. This approach also enables measurement of W-species surface diffusivity and domain growth rate as a function of growth conditions providing insight into the fundamental mechanisms of monolayer growth. Using this approach, coalesced monolayer and few-layer TMD films were obtained on sapphire substrates up to 2” in diameter at growth rates on the order of ~ 1 monolayer/hour. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to the sapphire resulting from a merging of an equal mixture of 0o and 60o oriented domains. WSe2 also grows epitaxially on hBN single crystal flakes but in this case the domains exhibit a predominant direction resulting in a reduced density of anti-phase boundaries. Applications and challenges of this approach in the growth of 2D heterostructures will also be discussed.

Temperature-dependent Raman spectroscopy studies of the interface coupling effect of monolayer ReSe2 single crystals on Au foils

Rhenium diselenide (ReSe2), which bears in-plane anisotropic optical and electrical properties, is of considerable interest for its excellent applications in novel devices, such as polarization-sensitive photodetectors and integrated polarization-controllers. However, great challenges to date in the controllable synthesis of high-quality ReSe2 have hindered its in-depth investigations and practical applications. Herein, we report a feasible synthesis of monolayer single-crystal ReSe2 flakes on the Au foil substrate by using a chemical vapor deposition route. Particularly, we focus on the temperature-dependent Raman spectroscopy investigations of monolayer ReSe2 grown on Au foils, which present concurrent red shifts of Eg-like and Ag-like modes with increasing measurement temperature from 77–290 K. Linear temperature dependences of both modes are revealed and explained from the anharmonic vibration of the ReSe2 lattice. More importantly, the strong interaction of ReSe2 with Au, with respect to that with SiO2/Si, is further confirmed by temperature-dependent Raman characterization. This work is thus proposed to shed light on the optical and thermal properties of such anisotropic two-dimensional three-atom-thick materials.

Emergence of a weak topological insulator from the BixSey family

We report the emergence of a weak topological insulator (WTI), BiSe, of the Bi-chalcogenide family with an indirect bandgap of 42 meV. Its structural unit consists of a bismuth bilayer (Bi2), a known quantum spin hall insulator sandwiched between two units of Bi2Se3 which are three dimensional strong topological insulators. Our density functional theory calculations confirm the WTI phase and angle resolved photo-emission spectroscopy measurements carried out on cleaved single crystal flakes show Rashba states that closely agree with our theoretical predictions. Finally, we present a comparison between electronic and magneto-transport properties measured on single crystal flakes and thin films of BiSe.

Advanced engineering of single-crystal gold nanoantennas

A nanofabrication process for realizing optical nanoantennas carved from a single-crystal gold plate is presented in this communication. The method relies on synthesizing two-dimensional micron-size gold crystals followed by the dry etching of a desired antenna layout. The fabrication of single-crystal optical nanoantennas with standard electron-beam lithography tool and dry etching reactor represents an alternative technological solution to focused ion beam milling of the objects. The process is exemplified by engineering nanorod antennas. Dark-field spectroscopy indicates that optical antennas produced from single crystal flakes have reduced localized surface plasmon resonance losses compared to amorphous designs of similar shape. The present process is easily applicable to other metals such as silver or copper and offers a design flexibility not found in crystalline particles synthesized by colloidal chemistry.

Electronic properties of germanane field-effect transistors

A new two dimensional (2D) material—germanane—has been synthesised recently with promising electrical and optical properties. In this paper we report the first realisation of germanane field-effect transistors fabricated from multilayer single crystal flakes. Our germanane devices show transport in both electron and hole doped regimes with on/off current ratio of up to 105(104) and carrier mobilities of 150 cm2 (V · s)−1(70 cm2 (V · s)−1) at K (room temperature). A significant enhancement of the device conductivity under illumination with nm red laser is observed. Our results reveal ambipolar transport properties of germanane with great potential for (opto)electronics applications.

Effect of grain boundaries on electrical properties of polycrystalline graphene

Since it was discovered, synthesizing large area graphene sheets by chemical or epitaxial methods is the conventional method for the application of graphene's outstanding properties. Unfortunately, CVD grown graphene is polycrystalline in nature with grain boundaries (GBs) unlike exfoliated single crystal graphene flakes. These GBs are known as the main source of degrading mechanical, chemical and electrical properties of CVD graphene. Nematic liquid crystal was employed to visualize grains and boundaries and to calculate the atomic orientation of adjacent grains. Electrical performance of various GB devices was observed, and the role of grain boundaries on the ultimate electrical performance was investigated. The same work was carried out at low temperature (∼7 K) to minimize the perturbation of two-dimensional electron which originated from the graphene lattice vibration. Interestingly, it was observed that there was no significant effect of the grain boundaries on the overall electrical performance of devices even at low temperature. These results lead to the conclusion that the grain boundaries in polycrystalline graphene sheets are not the cause for degradation in the electrical performance, if the grains were stitched well at the boundary in the growth process.

Gate-Tuned Thermoelectric Power in Black Phosphorus.

The electric field effect is a useful means of elucidating intrinsic material properties as well as for designing functional devices. The electric-double-layer transistor (EDLT) enables the control of carrier density in a wide range, which is recently proved to be an effective tool for the investigation of thermoelectric properties. Here, we report the gate-tuning of thermoelectric power in a black phosphorus (BP) single crystal flake with the thickness of 40 nm. Using an EDLT configuration, we successfully control the thermoelectric power (S) and find that the S of ion-gated BP reached +510 μV/K at 210 K in the hole depleted state, which is much higher than the reported bulk single crystal value of +340 μV/K at 300 K. We compared this experimental data with the first-principles-based calculation and found that this enhancement is qualitatively explained by the effective thinning of the conduction channel of the BP flake and nonuniformity of the channel owing to the gate operation in a depletion mode. Our results provide new opportunities for further engineering BP as a thermoelectric material in nanoscale.

Monolayer Single-Crystal 1T′-MoTe2 Grown by Chemical Vapor Deposition Exhibits Weak Antilocalization Effect

Growth of transition metal dichalcogenide (TMD) monolayers is of interest due to their unique electrical and optical properties. Films in the 2H and 1T phases have been widely studied but monolayers of some 1T'-TMDs are predicted to be large-gap quantum spin Hall insulators, suitable for innovative transistor structures that can be switched via a topological phase transition rather than conventional carrier depletion [ Qian et al. Science 2014 , 346 , 1344 - 1347 ]. Here we detail a reproducible method for chemical vapor deposition of monolayer, single-crystal flakes of 1T'-MoTe2. Atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy confirm the composition and structure of MoTe2 flakes. Variable temperature magnetotransport shows weak antilocalization at low temperatures, an effect seen in topological insulators and evidence of strong spin-orbit coupling. Our approach provides a pathway to systematic investigation of monolayer, single-crystal 1T'-MoTe2 and implementation in next-generation nanoelectronic devices.

Large Single Crystal SnS2 Flakes Synthesized from Coevaporation of Sn and S

Remarkable properties of layered metal dichalcogenides and their potential applications in various fields have raised intense interest worldwide. We report tens of microns-sized ultrathin single crystal SnS2 flakes grown on amorphous substrates using a simple one-step thermal coevaporation process. X-ray pole figure analysis reveals that a majority of flakes are oriented with the (0001) plane parallel to the substrate and a preferred fiber texture. For few-layer-thick SnS2, Moire patterns of 6-fold and 12-fold symmetries are observed by transmission electron microscopy imaging and diffraction. These patterns result from the relative rotation between SnS2 layers in the ultrathin flake. The 12-fold symmetry is consistent with a known quasicrystal pattern. The photoluminescence spectrum supports that these ultrathin flakes possess a direct bandgap. Carrier lifetime measured by time-resolved photoluminescence of a single flake is a few nanoseconds. These results improve our understanding of the formation and ...

Band gap engineering of a soft inorganic compound PbI2 by incommensurate van der Waals epitaxy

Van der Waals epitaxial growth had been thought to have trivial contribution on inducing substantial epitaxial strain in thin films due to its weak nature of van der Waals interfacial energy. Due to this, electrical and optical structure engineering via van der Waals epitaxial strain has been rarely studied. In this report, we show that significant band structure engineering could be achieved in a soft thin film material PbI2 via van der Waals epitaxy. The thickness dependent photoluminescence of single crystal PbI2 flakes was studied and attributed to the substrate-film coupling effect via incommensurate van der Waals epitaxy. It is proposed that the van der Waals strain is resulted from the soft nature of PbI2 and large van der Waals interaction due to the involvement of heavy elements. Such strain plays vital roles in modifying the band gap of PbI2. The deformation potential theory is used to quantitatively unveil the correlation between thickness, strain, and band gap change. Our hypothesis is confirmed by the subsequent mechanical bending test and Raman characterization.

Microstructure and Improved Coercivity of Mn&lt;sub&gt;1.33&lt;/sub&gt;Ga Nanoflakes by Surfactant-Assisted Ball Milling

In recent years, Mn x Ga alloys with tetragonal structure have attracted much attention because of the erratic fluctuation of rare earth supply and especially the tunable magnetic properties from fer-rimagnetic to ferromagnetic by varying Mn content. It is theoretically estimated that MnGa compound in L1 0 structure owes high anisotropy constant K u of 26 Mergs/cm3 and magnetization Ms of 845 emu/cm3, and thus can be regarded as a potential candidate for rare-earth-free permanent magnets [1-2]. At present, surfactant-assisted high-energy ball milling (HEBM) method has been regarded as one of the effective ways to realize high coercivity for magnetic materials with large anisotropy. It was reported [3-4] that high-aspect-ratio single-crystal or textured polynanocrystalline flakes of Sm-Co-based permanent magnetic materials with a submicron or nanosize thickness and high coercivity can be achieved. In this study, textured rare-earth-free Mn 1.33 Ga polycrystalline nanoflakes were prepared by surfactant-assisted HEBM in heptane with 15wt. % oleic acid (OA), and the effects of milling time on structure and magnetic properties were also investigated.