Although flakes of two-dimensional (2D) heterostructures at the micrometer scale can be formed with adhesive-tape exfoliation methods, isolation of 2D flakes into monolayers is extremely time consuming because it is a trial-and-error process. Controlling the number of 2D layers through direct growth also presents difficulty because of the high nucleation barrier on 2D materials. We demonstrate a layer-resolved 2D material splitting technique that permits high-throughput production of multiple monolayers of wafer-scale (5-centimeter diameter) 2D materials by splitting single stacks of thick 2D materials grown on a single wafer. Wafer-scale uniformity of hexagonal boron nitride, tungsten disulfide, tungsten diselenide, molybdenum disulfide, and molybdenum diselenide monolayers was verified by photoluminescence response and by substantial retention of electronic conductivity. We fabricated wafer-scale van der Waals heterostructures, including field-effect transistors, with single-atom thickness resolution.

In this work, we demonstrate the growth of HfSe2 thin films using molecular beam epitaxy. The relaxed growth criteria have allowed us to demonstrate layered, crystalline growth without misfit dislocations on other 2D substrates such as highly ordered pyrolytic graphite and MoS2. The HfSe2 thin films exhibit an atomically sharp interface with the substrates used, followed by flat, 2D layers with octahedral (1T) coordination. The resulting HfSe2 is slightly n-type with an indirect band gap of ∼ 1.1 eV and a measured energy band alignment significantly different from recent DFT calculations. These results demonstrate the feasibility and significant potential of fabricating 2D material based heterostructures with tunable band alignments for a variety of nanoelectronic and optoelectronic applications.

The limited grain size (<200 nm) for transition metal dichalcogenides (TMDs) grown by molecular beam epitaxy (MBE) reported in the literature thus far is unsuitable for high-performance device applications. In this work, the fundamental nucleation and growth behavior of WSe 2 is investigated through a detailed experimental design combined with on-lattice, diffusion-based first principles kinetic modeling to enable large area TMD growth. A three-stage adsorption-diffusion-attachment mechanism is identified and the adatom stage is revealed to play a significant role in the nucleation behavior. To limit the nucleation density and promote 2D layered growth, it is necessary to have a low metal flux in conjunction with an elevated substrate temperature. At the same time, providing a Serich environment further limits the formation of W-rich nuclei which suppresses vertical growth and promotes 2D growth. The fundamental understanding gained through this investigation has enabled an increase of over one order of magnitude in grain size for WSe 2 thus far, and provides valuable insight into improving the growth of other TMD compounds by MBE and other growth techniques such as chemical vapor deposition (CVD).

The interface between the topological insulator (TI) Bi 2 Se 3 and deposited metal films is investigated using X-ray photoelectron spectroscopy including conventional contact metals (Au, Pd, Cr, and Ir) and magnetic materials (Co, Fe, Ni, Co 0.8 Fe 0.2 , and Ni 0.8 Fe 0.2 ). Au is the only metal to show little or no interaction with the Bi 2 Se 3 , with no interfacial layer between the metal and the surface of the TI. The other metals show a range of reaction behaviors with the relative strength of reaction (obtained from the amount of Bi 2 Se 3 consumed during reaction) ordered as Au < Pd < Ir < Co ≤ CoFe < Ni < Cr < NiFe < Fe, in approximate agreement with the behavior expected from the Gibbs free energies of formation for the alloys formed. Post metallization anneals at 300 °C in vacuum were also performed for each interface. Several of the metal films were not stable upon anneal and desorbed from the surface (Au, Pd, Ni, and Ni 0.8 Fe 0.2 ), while Cr, Fe, Co, and Co 0.8 Fe 0.2 showed accelerated reactions with the underlying Bi 2 Se 3 , including interdiffusion between the metal and Se. Ir was the only metal to remain stable following anneal, showing no significant increase in reaction with the Bi 2 Se 3 . This study reveals the nature of the metal−Bi 2 Se 3 interface for a range of metals. The reactions observed must be considered when designing Bi 2 Se 3 -based devices.

The transfer-free direct growth of high-performance materials and devices can enable transformative new technologies. Here, room-temperature field-effect hole mobilities as high as 707 cm V s are reported, achieved using transfer-free, low-temperature (≤120 °C) direct growth of helical tellurium (Te) nanostructure devices on SiO /Si. The Te nanostructures exhibit significantly higher device performance than other low-temperature grown semiconductors, and it is demonstrated that through careful control of the growth process, high-performance Te can be grown on other technologically relevant substrates including flexible plastics like polyethylene terephthalate and graphene in addition to amorphous oxides like SiO /Si and HfO . The morphology of the Te films can be tailored by the growth temperature, and different carrier scattering mechanisms are identified for films with different morphologies. The transfer-free direct growth of high-mobility Te devices can enable major technological breakthroughs, as the low-temperature growth and fabrication is compatible with the severe thermal budget constraints of emerging applications. For example, vertical integration of novel devices atop a silicon complementary metal oxide semiconductor platform (thermal budget <450 °C) has been theoretically shown to provide a 10× systems level performance improvement, while flexible and wearable electronics (thermal budget <200 °C) can revolutionize defense and medical applications.

The growth of WTe2 thin films by molecular beam epitaxy is demonstrated for the first time on a variety of 2D substrates including MoS2, Bi2Te3, and graphite. We demonstrate that beam interruption of the metal source enables the growth of crystalline WTe2 films in the distorted octahedral (1T′) phase. As a result of the van der Waals nature of this material, a sharp interface between the WTe2 thin film and the substrate is shown with no evidence of misfit dislocations. In addition, the buckled structure expected for the semi-metallic 1T′ phase is observed with transmission electron microscopy. Raman spectroscopy further confirms the growth of the 1T′ phase and x-ray photoelectron spectroscopy shows the films to be stoichiometric and semi-metallic as expected.

Two-dimensional materials have shown great promise for implementation in next-generation devices. However, controlling the film thickness during epitaxial growth remains elusive and must be fully understood before wide scale industrial application. Currently, uncontrolled multilayer growth is frequently observed, and not only does this growth mode contradict theoretical expectations, but it also breaks the inversion symmetry of the bulk crystal. In this work, a multiscale theoretical investigation aided by experimental evidence is carried out to identify the mechanism of such an unconventional, yet widely observed multilayer growth in the epitaxy of layered materials. This work reveals the subtle mechanistic similarities between multilayer concentric growth and spiral growth. Using the combination of experimental demonstration and simulations, this work presents an extended analysis of the driving forces behind this non-ideal growth mode, and the conditions that promote the formation of these defects. Our study shows that multilayer growth can be a result of both chalcogen deficiency and chalcogen excess: the former causes metal clustering as nucleation defects, and the latter generates in-domain step edges facilitating multilayer growth. Based on this fundamental understanding, our findings provide guidelines for the narrow window of growth conditions which enables large-area, layer-by-layer growth.

The topologically protected surface states of three-dimensional (3D) topological insulators have the potential to be transformative for high-performance logic and memory devices by exploiting their specific properties such as spin-polarized current transport and defect tolerance due to suppressed backscattering. However, topological insulator based devices have been underwhelming to date primarily due to the presence of parasitic issues. An important example is the challenge of suppressing bulk conduction in BiSe and achieving Fermi levels ( E) that reside in between the bulk valence and conduction bands so that the topologically protected surface states dominate the transport. The overwhelming majority of the BiSe studies in the literature report strongly n-type materials with E in the bulk conduction band due to the presence of a high concentration of selenium vacancies. In contrast, here we report the growth of near-intrinsic BiSe with a minimal Se vacancy concentration providing a Fermi level near midgap with no extrinsic counter-doping required. We also demonstrate the crucial ability to tune E from below midgap into the upper half of the gap near the conduction band edge by controlling the Se vacancy concentration using post-growth anneals. Additionally, we demonstrate the ability to maintain this Fermi level control following the careful, low-temperature removal of a protective Se cap, which allows samples to be transported in air for device fabrication. Thus, we provide detailed guidance for E control that will finally enable researchers to fabricate high-performance devices that take advantage of transport through the topologically protected surface states of BiSe.