Ultrathin Crystals Research Articles

Unzipping Chemical Bonds of Non-Layered Bulk Structures to Form Ultrathin Nanocrystals

The rich electronic and band structures of monolayered crystals offer versatile physical/chemical properties and subsequently a wide range of applications. Fabrication, particularly by the top-down “exfoliation” processes, rely on the presence of weak Van der Waals force between individual layers. Due to the strong chemical bonds between planes and atoms, un-zipping ultra-thin crystals (from one to several unit cells thick) from non-layered structures is more challenging. This work reports a technique which can be used to prepare such ultra-thin crystals from bulk non-layered structures (ɑ-/β-MnO2, ZnO, TiO2, ɑ-TiB2). The physical and optical properties of these materials are characterized and contrasted against those from their bulk phases. The work presented here represents a tool kit for the preparation of novel 2D non-layered nanomaterials, providing significant contributions to this family of materials, paving the way for even more applications. Furthermore, we show the application of these novel NCs for bio-sensing and electrochemical oxygen reduction.

Sublimely simple method makes ultrathin organic crystals

Forming extremely thin crystals of organic semiconductors under ambient conditions is now possible. The key is a tiny gap. Material is sprayed on a heated surface where it sublimes and rises just 300 µm before condensing to form very thin crystals on the surface above ( 2020, DOI: ). The method uses a simple heater and common organic semiconductor materials to make ultrathin crystals, which have better performance in devices like light-emitting diodes and field-effect transistors because of the resulting film’s low electrical resistance. The method could make fabrication of these thin electronics easier and cheaper. Yang Liu, Xutang Tao, and their colleagues at Shandong University refined a method that they had developed to form pure organic crystals by simply vaporizing material across a gap of a few hundred micrometers ( 2018, DOI: ). The researchers used solid powders as the reservoir material in their first effort, but in the

Layer-dependent band gaps and dielectric constants of ultrathin fluorite crystals

Structure, stability, and thickness-dependent properties of ultrathin crystalline calcium fluoride (fluorite, CaF2) with promising application in two-dimensional (2D) field-effect transistors have been investigated by ab initio calculation. The enthalpy of formation and phonon calculations demonstrate the phase and dynamical stabilities of ultrathin CaF2. The verification of the stability for mono- and few-layer CaF2, which can be referred as idealized reference systems for ultrathin layers of CaF2 grown by CVD/MBE or other growth methods, indicates that 2D materials with thickness varying from monolayer to bulk limit can be obtained for CaF2 with hard-bond interlayer interaction. The strong chemical bonds between the adjacent layers in few-layer CaF2 are displayed, which is completely different from the weak interlayer van der Waals (vdW) interaction in layered MoS2 or graphite, resulting in the increased frequencies of low optical phonon modes due to the stronger interlayer restoring force compared to vdW layered crystals. Through the calculations of electronic structure and dielectric response, the layer-dependent band gaps and dielectric constants of ultrathin CaF2 are investigated, and the material can be designed by adjusting the layer thickness to fit practical applications. The large and tunable band gaps and superior dielectric properties of ultrathin CaF2, demonstrated from DFT-HSE06 and linear response calculations, signify the prospect of ultrathin CaF2 as an emerging 2D dielectric layer.

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.

Phase-controllable growth of ultrathin 2D magnetic FeTe crystals

Two-dimensional (2D) magnets with intrinsic ferromagnetic/antiferromagnetic (FM/AFM) ordering are highly desirable for future spintronic devices. However, the direct growth of their crystals is in its infancy. Here we report a chemical vapor deposition approach to controllably grow layered tetragonal and non-layered hexagonal FeTe nanoplates with their thicknesses down to 3.6 and 2.8 nm, respectively. Moreover, transport measurements reveal these obtained FeTe nanoflakes show a thickness-dependent magnetic transition. Antiferromagnetic tetragonal FeTe with the Néel temperature (TN) gradually decreases from 70 to 45 K as the thickness declines from 32 to 5 nm. And ferromagnetic hexagonal FeTe is accompanied by a drop of the Curie temperature (TC) from 220 K (30 nm) to 170 K (4 nm). Theoretical calculations indicate that the ferromagnetic order in hexagonal FeTe is originated from its concomitant lattice distortion and Stoner instability. This study highlights its potential applications in future spintronic devices.

A Synthetic Strategy of Ultrathin High-κ Antimony Oxide Single Crystals

Ultrathin antimony oxide single crystals with high dielectric constant and large breakdown electric field were synthesized via an expressly substrate-buffer-controlled chemical vapor deposition strategy. This strategy is also versatile for the synthesis of other ultrathin oxides.

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.

Controllable preparation of ultrathin 2D BiOBr crystals for high-performance ultraviolet photodetector

Ternary layered compound materials (bismuth oxyhalides and metal phosphorus trichalcogenides) stand out in electronic and optoelectronic fields due to their interesting physical properties. However, few studies focus on the preparation of high-quality two-dimensional (2D) BiOBr crystals with a typical layered structure, let alone their optoelectronic applications. Here, for the first time, high-quality 2D BiOBr crystals with ultrathin thicknesses (less than 10 nm) and large domain sizes (∼100 µm) were efficiently prepared via a modified space-confined chemical vapor deposition (SCCVD) method. It is demonstrated that a moderate amount of H2O molecules in the SCCVD system greatly promote the formation of high-quality 2D BiOBr crystals because of the strong polarity of H2O molecules. In addition, a linear relationship between the thickness of BiOBr nanosheets and Raman shift of $${\rm{A}}_{1{\rm{g}}}^{\left( 1 \right)}$$ mode was found. Corresponding theoretical calculations were carried out to verify the experimental data. Furthermore, the BiOBr-based photodetector was fabricated, exhibiting excellent performances with a responsivity of 12.4 A W−1 and a detectivity of 1.6×1013 Jones at 365 nm. This study paves the way for controllable preparation of high quality 2D BiOBr crystals and implies intriguing opportunities of them in op toelectronic applications.

ON COHERENT AND INCOHERENT SCATTERING OF FAST CHARGED PARTICLES IN ULTRATHIN CRYSTALS

We consider the fast charged particles scattering in ultrathin crystals on the base of the Born approximation of quantum electrodynamics. The main attention is paid to the question of the scattering cross section splitting into coherent and incoherent components when one of the crystallographic axes and planes is oriented along the direction of particle motion. It is shown that both the coherent and the incoherent components of the scattering cross section considerably depend on the orientation of the crystallographic axes relatively to the incident beam. In particular, it was shown that when particles are scattered by the crystal planes of atoms, the incoherent scattering cross section does not contain the Debye-Waller factor.

Ultrathin high-\u03ba antimony oxide single crystals

Ultrathin oxides have been reported to possess excellent properties in electronic, magnetic, optical, and catalytic fields. However, the current and primary approaches toward the preparation of ultrathin oxides are only applicable to amorphous or polycrystalline oxide nanosheets or films. Here, we successfully synthesize high-quality ultrathin antimony oxide single crystals via a substrate-buffer-controlled chemical vapor deposition strategy. The as-obtained ultrathin antimony oxide single crystals exhibit high dielectric constant (~100) and large breakdown voltage (~5.7 GV m−1). Such a strategy can also be utilized to fabricate other ultrathin oxides, opening up an avenue in broadening the applicaitons of ultrathin oxides in many emerging fields.

Nanometer-thick YIG-based magnonic crystals: Bandgap dependence on groove depth, lattice constant, and film thickness

We report on bandgap tuning in magnonic crystals made of nanometer-thick yttrium iron garnet (YIG) films with CoFeB-filled grooves via a variation of the groove depth, lattice constant, and film thickness. Using broadband spin-wave spectroscopy, we demonstrate bandgap widening in a 260-nm-thick YIG crystal when the grooves are deepened from half to full film thickness. Importantly, low-loss spin-wave transmission in the allowed bands of the magnonic crystal is almost unaffected by the patterning of fully discrete YIG stripes. Downscaling of the YIG film thickness to 35 nm decreases the bandgap size through a flattening of the spin-wave dispersion relation. We show that a reduction in the lattice constant effectively compensates for this trend. Our experimental results are corroborated by micromagnetic simulations, providing relevant information for the design of ultrathin YIG-based magnonic crystals with optimized bandgaps and spin-wave transmission properties.

Above Room Temperature Ferromagnetism in Gd2B2 Monolayer with High Magnetic Anisotropy

The realization of 2D ultrathin crystals with a ferromagnetic ground state that is sustainable at room temperature has been a real challenge now. By combining ab initio density functional theory wi...

Formation of ultrathin segregated-Ge crystal on Al/Ge(111) surface

We have investigated the formation of an ultrathin Ge crystal layer by vacuum annealing of the Al/Ge(111) structure. An hetero-epitaxial Al layer was found to be grown on a Ge(111) substrate by control of Al thickness and deposition rate during thermal evaporation, which the Al surface becomes the template of crystallographic structure of a segregated Ge layer. Then, surface flattening and Ge segregation of the Al/Ge(111) structure by vacuum annealing were evaluated, and the formation of ultrathin segregated-Ge crystals with a thickness below 1 nm on the hetero-epitaxial Al surface have been demonstrated.

Inverse transition of labyrinthine domain patterns in ferroelectric thin films.

Phase separation is a cooperative process, the kinetics of which underpin the orderly morphogenesis of domain patterns on mesoscopic scales1,2. Systems of highly degenerate frozen states may exhibit the rare and counterintuitive inverse-symmetry-breaking phenomenon3. Proposed a century ago4, inverse transitions have been found experimentally in disparate materials, ranging from polymeric and colloidal compounds to high-transition-temperature superconductors, proteins, ultrathin magnetic films, liquid crystals and metallic alloys5,6, with the notable exception of ferroelectric oxides, despite extensive theoretical and experimental work on the latter. Here we show that following a subcritical quench, the non-equilibrium self-assembly of ferroelectric domains in ultrathin films of Pb(Zr0.4Ti0.6)O3 results in a maze, or labyrinthine pattern, featuring meandering stripe domains. Furthermore, upon increasing the temperature, this highly degenerate labyrinthine phase undergoes an inverse transition whereby it transforms into the less-symmetric parallel-stripe domain structure, before the onset of paraelectricity at higher temperatures. We find that this phase sequence can be ascribed to an enhanced entropic contribution of domain walls, and that domain straightening and coarsening is predominantly driven by the relaxation and diffusion of topological defects. Computational modelling and experimental observation of the inverse dipolar transition in BiFeO3 suggest the universality of the phenomenon in ferroelectric oxides. The multitude of self-patterned states and the various topological defects that they embody may be used beyond current domain and domain-wall-based7 technologies by enabling fundamentally new design principles and topologically enhanced functionalities within ferroelectric films.

The effect of thickness on the optoelectronic properties of organic field-effect transistors: towards molecular crystals at monolayer limit

This manuscript reviews recent progress in the ultrathin monolayer molecular crystals (MMCs) for high performance optoelectronic devices.

Surficial Marangoni Flow‐Induced Growth of Ultrathin 2D Molecular Crystals on Target Substrates

Abstract2D molecular crystals (2DMCs) are emerging as excellent candidates for the application of organic field‐effect transistors (OFETs), owing to their enhanced charge transport efficiency and long‐range molecular ordering. However, many current methods involve an additional transfer process, which largely hinders the large‐area fabrication of high‐quality, intact 2DMCs for practical applications. Here, an efficient surficial Marangoni flow‐induced self‐assembly (SMFIS) method is developed for the fabrication of 2DMCs directly on target substrates without the need of a transfer process. This method utilizes a solvent/antisolvent system to confine the crystallization field at a liquid/air interface, and then the formed 2DMCs land on the target substrate with the full solvent evaporation. By controlling the vertical convection flow and the spreading coefficient of the solvent/antisolvent system, few‐layer (3–4 layers) 2DMCs are directly produced on the target substrates. The SMFIS method shows no substrate dependency and universal compatibility. With bis(triethylsilylethynyl)anthradithiophene (Dif‐TES‐ADT) as an example, the resulting 2D Dif‐TES‐ADT crystal exhibits an efficient charge transport with high average mobility up to 2.15 cm2 V−1 s−1. The proposed method with general applicability has great potential to be developed as a platform for the growth of high‐quality 2DMCs on the target substrate.

Guiding light at the nanoscale

Two recent papers demonstrate optical waveguides consisting of ultrathin photonic crystals, which approach the lowest possible thickness for such devices.

Synthesis and Optoelectronic Applications of a Stable p-Type 2D Material: α-MnS.

α-MnS, as a nonlayered p-type material with a wide band gap of 2.7 eV, has been expected to supplement the scarcity of two-dimensional (2D) p-type semiconductors, which are desperately required for constructing atomically thin p-n junctions. However, the preparation and property investigation of 2D α-MnS has scarcely been reported so far. Herein, we report the controlled synthesis of ultrathin large-scale α-MnS single crystals down to 4.78 nm via a facile chemical vapor deposition (CVD) method. Importantly, top-gating field-effect transistors based on the as-synthesized α-MnS nanosheets show p-type transport behavior with an ultrahigh on/off ratio exceeding 106, surpassing most reported p-type 2D materials. Meanwhile, α-MnS phototransistors exhibit an ultrahigh detectivity of 3.2 × 1014 Jones, as well as an excellent photoresponsivity of 139 A/W and a fast response time of 12 ms. Besides, outstanding environmental stability and admirable flexibility have also been demonstrated in the as-synthesized α-MnS nanosheets. We believe that this work broadens the scope of the CVD synthesis strategy for various p-type 2D materials and demonstrates their significant application potentials in electronics and optoelectronics.

Defect induced, layer-modulated magnetism in ultrathin metallic PtSe2.

Defects are ubiquitous in solids, often introducing new properties that are absent in pristine materials. One of the opportunities offered by these crystal imperfections is an extrinsically induced long-range magnetic ordering,1 a long-time subject of theoretical investigations.1–3 Intrinsic, two-dimensional (2D) magnetic materials4–7 are attracting increasing attention for their unique properties including layer-dependent magnetism4 and electric field modulation6. Yet, inducing magnetism into otherwise non-magnetic 2D materials remains a challenge. Here, we investigate magneto-transport properties of ultrathin PtSe2 crystals and demonstrate unexpected magnetism. Our electrical measurements show the existence of either ferromagnetic or anti-ferromagnetic ground state orderings depending on the number of layers in this ultra-thin material. The change in the device resistance upon application of a ~ 25 mT magnetic field is as high as 400 Ω with a magnetoresistance (MR) value of 5%. Our first-principles calculations suggest that surface magnetism induced by the presence of Pt vacancies and the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange couplings across ultra-thin films of PtSe2 are responsible for the observed layer-dependent magnetism. Considering the existence of such unavoidable growth-related vacancies in 2D materials, 8,9 these findings can expand the range of 2D ferromagnets into materials that would otherwise be overlooked.

Elastic Properties of 2D Ultrathin Tungsten Nitride Crystals Grown by Chemical Vapor Deposition

Abstract3D transition metal nitrides are well recognized for their good electrical conductivity, superior mechanical properties, and high chemical stability. Recently, 2D transition metal nitrides have been successfully prepared in the form of nanosheets and show potential application in energy storage. However, the synthesis of highly crystalline and well‐shaped 2D nitrides layers is still in demand for the investigation of their intrinsic physical properties. The present paper reports the growth of ultrathin tungsten nitride crystals on SiO2/Si substrates by a salt‐assisted chemical vapor deposition method. High‐resolution transmission microscopy confirms the as‐grown samples are highly crystalline WN. The stiffness of ultrathin WN is investigated by atomic force microscopy–based nanoindentation with the film suspended on circular holes. The 3D Young's modulus of few‐layer (4.5 nm thick or more) WN is determined to be 3.9 × 102 ± 1.6 × 102 GPa, which is comparable with the best experimental reported values in the 2D family except graphene and hexagonal boron nitride. The synthesis approach presented in this paper offers the possibilities of producing and utilizing other highly crystalline 2D transition‐metal nitride crystals.