Ultrathin Single Crystals Research Articles

Photophysical Properties of Submicrometer Ultrathin Perovskite Single-Crystal Films.

Organic-inorganic hybrid perovskite (OIHP) has attracted a great deal of interest with respect to diverse optoelectronic devices. However, the photophysical properties of the OIHP require further understanding because most of the investigations have been conducted with polycrystalline perovskites, which contain high-density structural defects. Here, diverse photophysical properties, including structural characterization, spectroscopic features, and photoexcited products, are studied in submicrometer CH3NH3PbBr3 ultrathin single-crystal (UTSC) films. Unlike polycrystalline films and large single crystals, the UTSC film provides a unique platform for studying spectroscopic characteristics of single-crystal perovskites. Compared with the polycrystalline film, the UTSC film presents an atomically flat morphology and near-perfect lattice with a lower defect density, leading to an isotropic system that can be applied in the construction of high-performance optoelectronic devices. Furthermore, a long lifetime emissive channel assigned to the trion is indicated, which is scarcely found in perovskite polycrystalline films. Our results profoundly improve our understanding of their photophysical properties and expand the horizons for perovskite materials in photonic applications.

Substrate Engineering toward Selective Growth of Ultrathin WC Crystals and Heterostructures via Liquid Cu‐Zn Catalyst

AbstractTransition metal carbides (TMCs) grown by chemical vapor deposition (CVD) offer promise for numerous novel phenomena and applications in the 2D limit. Despite considerable efforts thus far, the flexible customization of TMCs and their heterostructures still remains challenging. Herein, a substrate engineering is developed to achieve customized manufacturing of ultrathin WC single crystals and WC/graphene (WC‐G) heterostructures by varying the concentration of Zn in Cu‐Zn alloy substrate. It is worth noting that Zn atoms can remarkably reduce the nucleation density of graphene and promote the nucleation of WC. Thus, an increasing Zn content is applied to synergistically modulate the growth of graphene and WC, enabling the controllable fabrication of WC and WC‐G heterostructures. The synthesized WC crystals exhibit an ultrathin nature down to 3 nm, as well as high crystalline, ultra‐clean surface, and superb chemical stability. Based on that, the typical metallic properties with a temperature‐dependent resistance (nearly 1.30 Ω at 300 K and nearly 0.08 Ω at 1.7 K) and low resistance as well as excellent nonlinear optical performance of WC are demonstrated. This work provides fresh insights into regulating the growth behavior of multiblock‐structured carbides and promotes the study of their optic and electronic properties.

Supersymmetry Laser Arrays with High‐Order Exceptional Point

Supersymmetry (SUSY) laser array with superpartner structure can suppress excess modes to achieve high‐intensity and high‐coherent radiation. Compared with complex superpartner, using parity time (PT) symmetry broken to manipulate SUSY laser arrays is a more flexible approach. Herein, based on ultrathin perovskite single crystal, a SUSY laser array is constructed without superpartner, but with auxiliary gain–loss structure. PT symmetry broken with high‐order exceptional point (EP) is realized in the visible spectral range. It intrinsically avoids superpartner field mismatch and improves side mode suppression ratio 8.8–12 dB for single‐mode lasing. Furthermore, variations in EP‐order enable the transition from single‐mode to dual‐mode or tri‐mode radiation and retain amplified radiation characteristics. In addition, these SUSY laser arrays with the high‐order EP have the potential to be small‐volume optical sensor devices. The new design combining SUSY and PT symmetry broken presents its potential in micro‐nanophotonic devices with the benefit of small size, flexibility in spectral control, expandability, simplicity, and multifunctionality feature.

Controllable Oxidation of ZrS2 to Prepare High-κ, Single-crystal m-ZrO2 for 2D Electronics.

High-κ materials that exhibit large permittivity and band gaps are needed as gate dielectrics to enhance capacitance and prevent leakage current in downsized technology nodes. Among these, monoclinic ZrO2 (m-ZrO2 ) shows good potential because of its inertness and high-κ with respect to SiO2 , but a method to produce ultrathin single crystal is lacking. Here, the controllable preparation of ultrathin m-ZrO2 single crystals via the in situ thermal oxidation of ZrS2 is achieved. As-grown m-ZrO2 presents an equivalent oxide thickness of ≈0.29nm, a high dielectric constant of ≈19, and a breakdown voltage (EBD ) of ≈7.22 MV cm-1 . MoS2 field effect transistor (FET) by using m-ZrO2 as a dielectric layer shows comparable mobility to that using SiO2 dielectric. The ultraclean interface of m-ZrO2 /MoS2 and high crystalline quality of m-ZrO2 lead to negligible hysteresis in transfer curves. Single crystal m-ZrO2 dielectric shows potential application in digital complementary metal oxidesemiconductor (CMOS) logic FET.

Vortex glass phase transition in two-dimensional Bi2Sr2Ca2Cu3O sub-microbridge

Vortex dynamics is crucial for practical applications and to understand the nature of the mixed state for high-T c superconductors. Mechanically exfoliated ultra-thin single crystals provide a unique platform for exploring vortex physics in the two-dimensional (2D) limit. Here, we systematically investigated the current–voltage (I–V) characteristics as functions of the temperature and magnetic field in a single-crystalline Bi2223 sub-microbridge of 2.5 unit cells thickness. The nonlinear I–V characteristics are excellently described by the scaling theory for a quasi-2D vortex glass (VG) phase transition, and a phase diagram revealing the VG and vortex liquid phase is drawn. The scaling parameter v is consistent with previous reports, while the critical exponent z is far smaller than that in most investigations. Moreover, the VG transition temperature T g of the present sample is higher than that in the reported Bi2223 epitaxial thin films and tapes. In addition, the pinning force density of our sample is calculated, which is stronger than that reported in Bi2223 epitaxial thin films and tapes. Our results indicate that a high pinning force density may suppress the dynamical critical exponent z and enhance the VG phase transition temperature, providing new insight into the flux dynamics in cuprates.

Chemical Potential-Modulated Ultrahigh-Phase-Purity Growth of Ultrathin Transition-Metal Boride Single Crystals.

Two-dimensional (2D) transition-metal borides (TMBs) are especially expected to exhibit excellent performance in various fields among electricity, superconductivity, magnetism, mechanics, biotechnology, battery, and catalysis. However, the synthesis of ultrathin TMB single crystals with ultrahigh phase purity was deemed extremely challenging and has not been realized till date. That is because TMBs have the most kinds of crystal structures among inorganic compounds, which possess generous phase structures with similar formation energies compared with other transition-metal compounds, attributing to the metalloid and electron-deficient characteristics of boron. Herein, for the first time, we demonstrate a chemical potential-modulated strategy to realize the precise synthesis of various ultrahigh-phase-purity (approximately 100%) ultrathin TMB single crystals, and the precision in the phase formation energy can reach as low as 0.01 eV per atom. The ultrathin MoB2 single crystals exhibit an ultrahigh Young's modulus of 517 GPa compared to other 2D materials. Our work establishes a chemical potential-modulated strategy to synthesize ultrathin single crystals with ultrahigh phase purity, especially those with similar formation energies, and undoubtedly provides excellent platforms for their extensive research and applications.

Individual and synergetic charge transport properties at the solid and electrolyte interfaces of a single ultrathin single crystal of organic semiconductors.

The chemical structures and morphologies of organic semiconductors (OSCs) and gate dielectrics have been widely investigated to improve the electrical performances of organic thin-film transistors (OTFTs) because the charge transport therein is a phenomenon at the semiconductor-dielectric interfaces. Here, solid and ionic gel gate dielectrics were adopted on the lower and upper surfaces, respectively, of a single, two molecule-thick single crystals of p-type OSCs to study the charge transport properties at individual interfaces between the morphologically compatible OSC surface and different gate dielectrics. Using the four-probe method, the solid and ionic gel interfaces were found to exhibit hole mobilities of 9.3 and 2.2 cm2 V-1 s-1, respectively, which revealed the crucial impact of the gate dielectric materials on the interfacial charge transport. Interestingly, when gate biases are applied through both dielectrics, i.e., under the solid/ionic gel dual-gate transistor operation, the hole mobility at the solid gate interface is improved up to 14.7 cm2 V-1 s-1, which is 1.5 times greater than that assessed without the ionic gel gate. This improvement can be attributed to the electric double layer formed at the ionic gel/uniform crystal surface, which provides a close-to-ideal charge transport interface through dramatic trap-filling. Therefore, the present dual-gate transistor technique will be promising for investigating the intrinsic charge-transport capabilities of OSCs.

Fabrication and transport properties of two dimensional Bi2Sr2Ca2Cu3O10+δ micro-bridge

Ultra-thin high-temperature superconducting films have attracted continuous interest due to their potential electronic applications, which also provide a unique platform of novel physics and properties in the two-dimensional limit. We, here, realized fabrication of two-unit-cell-thick micro-bridges from mechanically exfoliated ultra-thin Bi2Sr2Ca2Cu3O10+δ (Bi2223) single crystals and systematically investigated their transport properties. The two-dimensional superconducting nature is verified by the existence of the Berezinskii–Kosterlitz–Thouless transition, which is simultaneously revealed by current-voltage properties and the zero-field temperature dependence of resistance. Comparing with Bi2223 bulk crystal, a Bi2223 micro-bridge shows a slight lower upper critical field but pronounced improvement in the critical current density. Our findings indicate that the ultra-thin Bi2223 single crystal is highly prospective for both scientific investigations of unconventional superconductivity and applications of high Tc superconducting devices.

Toward Microlasers with Artificial Structure Based on Single-Crystal Ultrathin Perovskite Films.

A perovskite microlaser is potentially valuable for integrated photonics due to its excellent properties. The artificial microlasers were mostly made on polycrystalline films. Though a perovskite single crystal has significantly improved properties in comparison with its polycrystalline counterpart, an artificial microlaser based on single-crystal perovskite has been much less explored due to the difficulty in producing an ultrathin-single-crystal (UTSC) film. Here we show a device processing based on a perovskite UTSC film, confirming the high performance of the UTSC device with a quality factor of 1250. The single-crystal device shows 4.5 times the quality factor and 8 times the radiation intensity in comparison with its polycrystalline counterpart. The experiment first proved that hybrid perovskite microlasers with a subwavelength fine structure can be processed by focused ion beams (FIB). In addition, a wavelength-tunable distributed feedback (DFB) laser is demonstrated, with a tuning range of ∼4.6 nm. The research provides an easily applicable approach for perovskite photonic devices with excellent performance.

A versatile sample fabrication method for ultrafast electron diffraction

Integral to the exploration of nonequilibrium phenomena in solid-state systems is the study of lattice motion after photoexcitation by a femtosecond laser pulse. For the past two decades, ultrafast electron diffraction (UED) has played a critical role in this regard. Despite remarkable progress in instrumental development, this technique is still bottlenecked by a demanding sample preparation process, where ultrathin single crystals of large lateral size are typically required. In this work, we describe an efficient, versatile method that yields high-quality, laterally extended (≥ 100 µm), and thin (≤ 50 nm) single crystals on amorphous films of Si3N4 windows. It applies to most exfoliable materials, including those reactive in ambient conditions, and promises clean, flat surfaces. Besides the natural extension to fabricating van der Waals heterostructures, our method can also be applied to future-generation UED that enables additional control of sample parameters, such as electrostatic gating and excitation by a locally enhanced terahertz field. Our work significantly expands the type of samples for UED studies and also finds application in other time-resolved techniques such as attosecond extreme-ultraviolet absorption spectroscopy. This method hence provides further opportunities to explore photoinduced transitions and to discover novel states of matter out of equilibrium.

Cesium manganese chloride: Stable lead-free perovskite from bulk to single layer

Motivated by the recent advances in perovskite-based solar cells, here we investigate stability, electronic properties and vibrational characteristics of lead-free perovskite, CsMnCl3, and its low dimensional forms by means of first-principles calculations. Structural optimizations reveal that, regardless of whether it is bulk or ultra-thin single layer cubic perovskite structure, CsMnCl3 crystal exhibit robust antiferromagnetism in its ground state due to oppositely aligned magnetic moments of Mn atoms. In addition to total energy calculations, phonon band dispersions indicate that CsMnCl3 structure sustains its dynamical stability down to its thinnest single layer crystal structures. The calculated Raman spectrums state that while the first-order Raman scattering is forbidden for bulk CsMnCl3 due to the cubic symmetry; dimensional-reduction-driven symmetry breaking leads to emergence of experimentally-observable distinctive Raman active modes in bilayer and single-layer crystal structures. Moreover, the electronic band dispersions reveal that from its bulk to ultra-thin single layer structures CsMnCl3 crystals are robust antiferromagnetic insulators. Multiple valid features like controllable dimensionality, robust antiferromagnetism and wide electronic band gap make cubic CsMnCl3 crystal as a potential candidate for nano-scale optoelectronic applications.

Controlled growth of 2D ultrathin Ga2O3 crystals on liquid metal.

2D metal oxides (2DMOs) have drawn intensive interest in the past few years owing to their rich surface chemistry and unique electronic structures. Striving for large-scale and high-quality novel 2DMOs is of great significance for developing future nano-enabled technologies. In this work, we demonstrate for the first time controllable growth of highly crystalline 2D ultrathin Ga2O3 single crystals on liquid Ga by the chemical vapor deposition approach. With the introduction of oxygen into the growth process, large-area hexagonal α-Ga2O3 crystals with a uniform size distribution have been produced. At high temperature, fast diffusion of oxygen atoms onto the liquid surface facilitates reaction with Ga and thus leads to in situ formation of 2D ultrathin crystals. By precisely controlling the amount of oxygen, the vertical growth of the Ga2O3 single crystal has been realized. Furthermore, phase engineering can be achieved and thus 2D β-Ga2O3 crystals were also prepared by precisely tuning the growth temperature. The controlled growth of 2D Ga2O3 crystals offers an applicable avenue for fabrication of other 2D metal oxides and can further open up possibilities for future electronics.

Growth of 2D MoP single crystals on liquid metals by chemical vapor deposition

Two-dimensional (2D) transition metal phosphides (TMPs) are predicted with many novel properties and various applications. As a member of TMPs family, molybdenum phosphide (MoP) exhibits many exotic physicochemical properties. However, the synthesis of high-quality 2D MoP single crystals is not reported due to the lack of reliable fabrication method, which limits the exploration of 2D MoP. Here, we report the growth of high-quality ultrathin MoP single crystals with thickness down to 10 nm on liquid metals via chemical vapor deposition (CVD). The smooth surface of liquid Ga is regarded as a suitable growth substrate for producing 2D MoP single crystals. The Mo source diffuses toward the Ga surface due to the high surface energy to react with phosphorus source, thus to fabricate ultrathin MoP single crystals. Then, we study the second harmonic generation (SHG) of 2D MoP for the first time due to its intrinsic non-centrosymmetric structure. Our study provides an new approach to synthesize and explore other 2D TMPs for future applications.

Universal growth of ultra-thin III\u2013V semiconductor single crystals

Ultra-thin III–V semiconductors, which exhibit intriguing characteristics, such as two-dimensional (2D) electron gas, enhanced electron–hole interaction strength, and strongly polarized light emission, have always been anticipated in future electronics. However, their inherent strong covalent bonding in three dimensions hinders the layer-by-layer exfoliation, and even worse, impedes the 2D anisotropic growth. The synthesis of desirable ultra-thin III–V semiconductors is hence still in its infancy. Here we report the growth of a majority of ultra-thin III–V single crystals, ranging from ultra-narrow to wide bandgap semiconductors, through enhancing the interfacial interaction between the III–V crystals and the growth substrates to proceed the 2D layer-by-layer growth mode. The resultant ultra-thin single crystals exhibit fascinating properties of phonon frequency variation, bandgap shift, and giant second harmonic generation. Our strategy can provide an inspiration for synthesizing unexpected ultra-thin non-layered systems and also drive exploration of III–V semiconductor-based electronics.

Trap-state suppression and band-like transport in bilayer-type organic semiconductor ultrathin single crystals

Fundamental study of the carrier transport in both the channel layer and the electrode-channel contact of organic semiconductor crystals is indispensable for achieving high-performance organic field-effect transistors. In this paper, we report the temperature-dependent carrier transport of high-mobility organic semiconductors called $2\text{--}\text{decyl}\text{--}7\text{-phenyl-}[1]\text{benzothieno}[3,2\text{--}b][1]\mathrm{benzothiophene}$ (Ph-BTBT-C10) and $3\text{-decyl-}9\text{-phenyl-}[1]\text{benzothieno}[3,2\text{--}b]\text{naphtho}[2,3\text{--}b]\text{thiophene}$ (Ph-BTNT-C10). The use of single-crystal films with controlled bilayer-number thickness enabled the simultaneous study of intralayer and interlayer transport at cryogenic temperatures. Four-probe measurement of two-bilayer- and three-bilayer-thick films of Ph-BTBT-C10 suggests that the access resistance is dominated by tunneling transport across the insulating alkyl-chain layers. Single-crystal thin-film transistors of these materials showed band-like carrier transport down to 80 K and the carrier mobility of Ph-BTBT-C10 reached $34\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$. Detailed analysis of the low-temperature characteristics revealed small activation energy of approximately 5 meV and a sharp distribution of band tail states. These findings suggest that high crystallinity owing to the bilayer-type crystal structure effectively suppresses the localization of gate-induced carriers.

Oriented Growth of Ultrathin Single Crystals of 2D Ruddlesden-Popper Hybrid Lead Iodide Perovskites for High-Performance Photodetectors.

Compared with their 3D counterparts, layered 2D Ruddlesden-Popper perovskites (2D RPPs) with the formula of (A')2A n-1Pb nX3 n+1 exhibit improved photo and moisture stability. To develop flexible single-crystalline electronics and wearable devices, cost-effective growth of large-area single crystals of 2D RPPs with large lateral size, ultrathin thickness, parallel orientation, and well-defined and controlled n values are highly desirable, but it remains still a challenge. Here, we modified the space-confined aqueous solution growth method to fabricate single-crystalline films of (BA)2(MA) n-1Pb nX3 n+1 and n = 1, 2, 3, respectively, with the lateral size reaching millimeter and thickness about 400-1000 nm. Moreover, the quantum well layers are found to be parallel to the substrate, well suitable for lateral transport requirement. We successfully integrated ultrathin single crystals of (BA)2(MA) n-1Pb nX3 n+1 ( n = 1-3) into a metal-semiconductor-metal two-terminal photodetector. The photoresponsive wavelength range was extended from 525 nm for n = 1 and 590 nm for n = 2 to 630 nm for n = 3 as a result of the reduced exciton band gap. The device of n = 2 single crystals shows the highest responsivity of 2.96 A/W and a detectivity of 1013 jones, while the device of n = 3 shows the lowest dark current 10-14 A, and therefore, its on/off ratio reaches 2862 at the bias voltage of 1 V.

Self-Powered X-Ray Photodetector Based on Ultrathin PbI2Single Crystal

Lead iodide (PbI2) single crystals have been extensively investigated due to their potential applications in nuclear radiation detectors at room temperature. In this letter, the properties of ultrathin PbI2 single crystals were obtained and investigated by X-ray diffraction, ultraviolet-visible diffuse reflection spectroscopy, and field emission scanning electron microscopy. The X-ray photodetector based on ultrathin PbI2 single crystal with a Schottky configuration of Au/PbI2/Mo was also fabricated through a facile method. Especially, the X-ray photodetector exhibits good stability and reproducibility. A large sensitivity of ${0}.{12} \times {10}^{{4}} \mu \text{C}$ /Gyair/cm2 was obtained for X-ray photodetector based on PbI2 single crystal at zero bias under a low radiation dose rate (0.19 mGy/h), which is sixty times higher than that of currently $\alpha $ -Se X-ray detectors at high bias. Moreover, we have further illustrated the working principles of the photodetector at zero, positive and negative bias with and without X-ray illumination. Our investigation substantiates the potential of the self-powered X-ray photodetector based on ultrathin PbI2 single crystal as a candidate device in optoelectronic applications.

Transport Properties of Topological Semimetal Tungsten Carbide in the 2D Limit

AbstractRecent theoretical calculations and spectroscopy measurements have shown that several materials with the tungsten carbide‐type structure, such as molybdenum phosphide and tungsten carbide (WC), are a new type of topological semimetal hosting triply degenerate nodal points. So far, most of experimental studies are performed on large‐size bulk crystal. 2D nanostructures, with large surface‐to‐volume ratio, are expected to provide an attractive system for probing topological surface states and the development of device applications. Here the transport characterization of high‐quality ultrathin WC single crystals is reported. The magnetoresistance (MR) shows a strong anisotropic behavior, which may be attributed to the transport anomalies of three‐component fermions as theoretically proposed. The scaling analysis reveals the presence of the surface states in WC. The observation of the nonsaturating MR in strong magnetic fields, together with nonlinear and strong temperature dependent Hall effect, is consistent with the proposed multiple bands character of carriers in WC. In addition, a low‐temperature upturn in the resistance is observed, indicating that electron–electron interaction plays a prominent role in charge transport. The results demonstrate that the ultrathin WC crystals offer a potential platform for exploring exotic transport properties arising from the three‐component fermions in the 2D limit.

Ultrathin Non‐van der Waals Magnetic Rhombohedral Cr2S3: Space‐Confined Chemical Vapor Deposition Synthesis and Raman Scattering Investigation

AbstractTwo dimensional (2D) magnetic materials display enormous application potential in spintronic fields. However, most of currently reported magnetic materials are van der Waals layered structure that is easy to be isolated via exfoliation method. By contrast, the studies on non‐van der Waals ultrathin magnetic materials are rare, largely due to the difficulty in fabrication. Rhombohedral Cr2S3, an intensively studied antiferromagnetic transition metal chalcogenide with Neel temperature of ≈120 K, has a typical non‐van der Waals structure. Restricted by the strong covalent bonding in all the three dimensions of non‐van der Waals structure, the synthesis of ultrathin Cr2S3 single crystals is still a challenge that is not achieved yet. Besides, the study on the Raman modes of rhombohedral Cr2S3 is also absent. Herein, by employing space‐confined chemical vapor deposition strategy, ultrathin rhombohedral Cr2S3 single crystals with a thickness down to ≈2.5 nm for the first time are successfully grown. Moreover, a systematically investigation is also conducted on the Raman vibrations of ultrathin rhombohedral Cr2S3. With the aid of angle‐resolved polarized Raman technique, the Raman modes of rhombohedral Cr2S3 for the first time based on crystal symmetry and Raman selection rules are rationally assigned.

Ultrathin 2D GeSe2 Rhombic Flakes with High Anisotropy Realized by Van der Waals Epitaxy

AbstractAs a layered p‐type semiconductor with a wide bandgap of 2.7 eV, GeSe2 can compensate for the rarity of p‐type semiconductors, which are desired for the production of high‐integration logic circuits with low power consumption. Herein, ultrathin 2D single crystals of β‐GeSe2 are produced using van der Waals epitaxy and halide assistance; each crystalline flake is ≈7 nm thick and shaped as a rhombus. The optical and electrical properties of the flakes are studied systematically, and the temperature‐dependent Raman spectra of the flakes reveal that the intensity of the Raman peaks decrease with increasing temperature. Low‐temperature electrical measurements suggest that the variable‐range hopping model is best for describing the electrical transport at 20–180 K; meanwhile, optical‐phonon‐assisted hopping can account for the transport behavior at 180–460 K. Impressively, the angle‐resolved polarized Raman measurements indicate strong in‐plane anisotropy of the rhombic GeSe2 flake under a parallel polarization configuration, which may result from the low symmetry of the monoclinic crystal structure of GeSe2. Furthermore, a photodetector based on a rhombic GeSe2 flake is constructed and shown to exhibit a high responsivity of 2.5 A W−1 and a fast response of ≈0.2 s.