Session G1: Focus Session: Beyond Graphene - Growth I

An In-Plane Epitaxial Heterostructure of Two-Dimensional Crystals

By adapting the concept of epitaxy to two-dimensional (2D) space, a single-atomic-layer, in-plane heterostructure of a prototypical material system, graphene and hexagonal boron nitride (h-BN), has been grown. It is shown by multiple complementary experimental techniques that monolayer crystalline h-BN grows from fresh edges of monolayer graphene with lattice coherence, forming an abrupt 1D interface. The challenges of obtaining truly 2D heterostructures with lattice coherence and sharp interface unassisted by templates in the third dimension will be discussed. Importantly, the h-BN lattice orientation is solely determined by the graphene, forgoing configurations favored by the supporting substrate, a polycrystalline Cu foil with an exclusively (100) surface. To illustrate this important feature of heteroepitaxy in 2D, this talk will briefly discuss the graphene/Cu(100) and h-BN/Cu(100) orientational relations when the two materials are grown alone on Cu foils. For a counterintuitive reason, when grown alone, h-BN strictly aligns to Cu(100) exhibiting four and only four symmetrically equivalent orientations, while graphene shows a wide spread of rotations. The energetically favored h-BN/Cu(100) orientational alignment is overridden when h-BN is grown as an ``epistrip'' templated by a graphene edge. This talk will allude to the interesting physics of the 1D boundary states that has been theoretically predicted, such as spin polarization. As an intermediate step towards establishing the long-predicted physical properties, the boundary states have been observed by atomic-resolution scanning tunneling microscopy and tunneling spectrum mapping, although the sought-after spin polarization is destroyed by the presence of the Cu substrate.\\[4pt] This work was partially supported by DARPA (approved for public release; distribution is unlimited) and NSF (ECCS-1231808). A portion of this research was conducted at the Center for Nanophase Materials Sciences (CNMS), sponsored at Oak Ridge National Laboratory through the Scientific User Facilities Division, by Office of Basic Energy Sciences, US Department of Energy (DOE), who also supported the work at Sandia under contract DE-AC04-94AL85000. Support received by contributors to the work also includes: Natural Science Foundation of China (Grants 11034006 and 11204286), National Key Basic Research Program of China (Grant 2014CB921103), and the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory. Computational resources at the National Energy Research Scientific Computing Center of the US DOE were used.   

Wafer-Scale Monolayer Films of Semiconducting Metal Dichalcogenides for High-Performance Electronics

Two-dimensional semiconducting transition metal dichalcogenides (TMDs) have shown their potential in electronics, optoelectronic and valleytronis. However, large-scale growth methods reported to date have only produced materials with limited structural and electrical uniformity, hindering further technological applications. Here we present a 4-inch scale growth of continuous monolayer molybdenum disulfide (MoS2) and tungsten disulfide (WS2) films that show excellent structural and electrical uniformity over the entire wafer using metal-organic chemical vapor deposition. The resulting monolayer films show high mobility of 30 cm2/Vs at room temperature, as well as the phonon-limited transport for MoS2, regardless of the channel length and device location. They allow for the batch fabrication of monolayer MoS2 field effect transistors with a 99{\%} yield, which display spatially-uniform n-type transistor operation with a high on/off ratio. We further demonstrate the multi-level growth and fabrication of vertically-stacked monolayer MoS2 films and devices, which could enable the development of novel three-dimensional circuitry and device integration.   

Synthesis and Investigation of van der Waals Heterostructures

The recent isolation of single layers of transition metal dichalcogenides (TMD) has demonstrated that reducing dimensionality can alter the material properties. In particular, MoS$_{\mathrm{2}}$, MoSe$_{\mathrm{2}}$, WS$_{\mathrm{2}}$, and WSe$_{\mathrm{2}}$ exhibit an abrupt transition from indirect to direct bandgap semiconductors at monolayer thickness. Monolayer TMDs are promising materials for electronic components due to their high mobility, high on/off ratio, and low standby power dissipation. Additionally, selective layer-by-layer stacking to form van der Waals (vdW) heterostructures may provide the ability to controllably engineer electronic, optic, and spintronic properties. Recently, several methods were investigated to achieve vdW heterostructures including sequential exfoliation, stacking of chemical vapor deposition (CVD) grown monolayers, and epitaxial growth of bilayers. We detail our CVD synthesis of the monolayer TMDs (MoS$_{\mathrm{2}}$, MoSe$_{\mathrm{2}}$, WS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}})$ and the subsequent fabrication and characterization of vdW heterostructures. In our heterostructures, we observe a dramatic decrease in PL intensity compared to the monolayer constituents. The Raman spectra exhibit clear and distinct differences from a superposition of monolayer spectra, demonstrating that interactions across the van der Waals interface in these heterostructures may significantly modify the net electronic properties. We find the observed behaviors are influenced by many factors, including charge transfer, substrate effects, stacking sequence, as well as intra- and inter-layer exciton formation, which will be discussed here.   

Large Area Growth and Characterization of Monolayer MoS$_{2}$

Two-dimensional transition metal dichalcogenides have garnered extensive attention due to their direct band gap with great potential in semiconductor application complementing graphene. While most of the experiments were carried out on either exfoliated films or CVD grown crystals, sample size are restricted in hundreds of micrometers. Synthesis of large-area samples were less successful. Here, we report the growth of cm$^{2}$-scale molybdenum disulfide (MoS$_{2})$ monolayer with a facile method by sulfurizing molybdenum trioxide film on sapphire substrates. Uniformity and quality of the monolayer films were verified by Raman, PL mapping and PL efficiency. A quasi- molten phase of the precursor in the initial stage of the reaction is found to be crucial for the monolayer growth.   

Direct growth of hexagonal boron nitride on epitaxial graphene

In this work, we demonstrate recent attempts at achieving the direct growth of hexagonal boron nitride (h-BN) on epitaxial graphene. By exposing our graphene samples (grown on Si-face SiC) to a low-pressure ($\sim$ 1$\times$10$^{-4}$ Torr) background of borazine at temperatures exceeding 1000$^{\circ}$C, we obtain \emph{in-situ} low-energy electron diffraction patterns consistent with the presence of many randomly oriented grains of h-BN. We find that increasing the growth temperature leads to the development of a preferential orientation, with the h-BN aligning with the underlying SiC substrate. Atomic-force microscopy and low-energy electron microscopy (LEEM) show triangular crystals exceeding 1 $\mu$m in extent. Additionally, using a first-principles method for examining low-energy electron reflectivity spectra,\footnote{R. M. Feenstra et al., Phys. Rev. B 87, 041406(R) (2013).} we are capable of determining the coverage of h-BN on our samples. We show that our method is sufficiently robust to discriminate between various combinations of numbers of h-BN monolayers (MLs) and graphene MLs based on unique features in their spectra. Prospects for improvement of the h-BN crystallinity, as well as the controlled growth of a specific number of MLs are discussed.   

Chemically controlled dislocation migration in two-dimensional MoS$_{2}$

As an additional Gibbs degree of freedom, chemical potential plays an important role in determining various properties of hetero-elemental two-dimensional (2D) materials, ranging from the equilibrium shape [1], to defect structures [2,3], electronic [3], and magnetic properties [4]. Here, by first-principles calculations, we demonstrate how the chemical potential control can be adopted as a feasible strategy to tune the dislocation dynamics in 2D MoS$_{2}$. Depending on the structures of the migrating dislocations, two different dynamic mechanisms are revealed, either through the direct \textit{bond-rebinding (BR)} mechanism, where only single metal atom moves significantly, or \textit{concerted migration (CM)}, in which case several atoms rearrange concurrently. The migration barriers for CM mechanism could be 2 to 4 times larger as the BR ones. Our detailed analyses show that under certain range of chemical potential some dislocations with high mobility could nevertheless also have quite high stability, which opens up the possibility of enhanced plasticity and suggests intriguing future applications. \\[4pt] [1] Y. Liu, X. Zou, and B. I. Yakobson, \textit{ACS Nano}, \textbf{6}, 7053-7058 (2012). \\[0pt] [2] W. Zhou*, X. Zou*, S. Najmaei, et al., \textit{Nano Lett.}, \textbf{13}, 2615-2522 (2013). \\[0pt] [3] X. Zou, Y. Liu, and B. I. Yakobson, \textit{Nano Lett.}, \textbf{13}, 253-258 (2013). \\[0pt] [4] Z. Zhang, X. Zou, V. H. Crespi, and B. I. Yakobson, \textit{ACS Nano}, \textbf{7}, 10475-10481 (2013).   

Growth, transfer, structural, optical, and electrical properties of large-size transition-metal dichalcogenide monolayer single-crystals

We report growth, transfer process, as well as structural, optical, and electrical properties of large-size and high-quality two-dimensional transition-metal dichalcogenide MX2 (M=Mo, W; X=S, Se) single-crystalline triangular-shape nanosheets composed of one to a few monolayers. A vapor-trapping enhanced chemical vapor deposition approach was employed for the MX2 monlayer single crystal growth. The number of layers, crystallinity, and uniformity of the as-grown MX2 were characterized and confirmed by Raman and photoluminescence measurements. The MX2 monlayer single-crystal triangles show comparable size and uniformity to the state-of-the-art results reported as of now. The optical properties of the MX2 were studied based on the analysis of the photoluminescence results. The electrical properties including resistivity, mobility, carrier type and concentration, and contact resistance of the MX2 were characterized by both three-terminal field-effect transistor and Hall effect transport measurements. The Hall bar devices were fabricated by lithography and dry-etching of the as-grown single-crystalline MX2. The transfer process of the MX2 from growth substrate (SiO2-on-Si) to various substrates was successfully demonstrated.   

Vacancy-induced growth of inversion domains in transition-metal dichalcogenide monolayer: an atomic view of defect dynamics

In this work, we systematically study the nucleation and growth of inversion domains within a monolayer MoSe$_{2}$. We use a focused electron beam to generate and excite Se vacancies in the monolayer, and monitor their dynamics through sequential atomic-scale Z-contrast imaging. We find that Se vacancies are first randomly created and then preferentially agglomerate into line defects under the energy transferred from the electron beam. Successive evolution of these line defects induces nucleation of distinct triangular inversion domains within the MoSe$_{2}$ layer and generates conductive 60$^{\circ}$ grain boundaries within the semiconducting matrix. Density functional theory shows that the nucleation of inversion domain lowers the system energy due to the release of vacancy-induced lattice shrinkage. Migration of the grain boundaries can be further activated by deformation of the peripheral lattice, giving rise to the growth of the inversion domain. The grain nucleation and growth process observed under energy transferred from the electron beam can provide new insights into the structural stability of TMDC monolayers under severe (e.g. high temperature) working conditions.   

Development of Novel Two-dimensional Layers, Alloys and Heterostructures

The one-atom-think graphene has fantastic properties and attracted tremendous interests in these years, which opens a window towards various two-dimensional (2D) atomic layers. However, making large-size and high-quality 2D layers is still a great challenge. Using chemical vapor deposition (CVD) method, we have successfully synthesized a wide varieties of highly crystalline and large scale 2D atomic layers, including h-BN, metal dichalcogenides e.g. MoS$_{2}$, WS$_{2}$, CdS, GaSe and MoSe$_{2}$ which belong to the family of binary 2D materials. Ternary 2D alloys including BCN and MoS$_{\mathrm{2x}}$Se$_{\mathrm{2(1-x)}}$ are also prepared and characterized. In addition, synthesis of 2D heterostructures such as vertical and lateral graphene/h-BN, vertical and lateral TMDs are also demonstrated. Complementary to CVD grown 2D layers, 2D single-crystal (bulk) such as Phosphorene (P), WTe$_{2}$, SnSe$_{2}$, PtS$_{2}$, PtSe$_{2}$, PdSe$_{2}$, WSe$_{\mathrm{2x}}$Te$_{\mathrm{2(1-x)}}$, Ta$_{2}$NiS$_{5}$ and Ta$_{2}$NiSe$_{5}$ are also prepared by solid reactions. There work provide a better understanding of the atomic layered materials in terms of the synthesis, atomic structure, alloying and their physical properties. Potential applications of these 2D layers e.g. optoelectronic devices, energy device and smart coating have been explored.   

The kinetics of white graphene (h-BN) growth on the planarized Ni foil surfaces

Morphology of the surface and the grain orientation of the metal catalysts have been considered as two important factors for the growth of h-BN by a CVD method. We report correlation between growth rate of h-BN and orientation of nickel grains. The surface of the nickel foil was first planarized by electrochemical polishing and subsequently annealed in atmospheric pressure hydrogen to suppress the effect from the surface morphology. The atmospheric annealing with hydrogen reduced nucleation site of h-BN such that large crystal size mainly has grown from the grain boundary and few other nucleation sites in the nickel foil. Higher growth rate of h-BN was observed from the nickel grains that has \textbraceleft 110\textbraceright or \textbraceleft 100\textbraceright orientation due to higher surface energy.   

Nanoscale topographical replication of graphene architecture by manufactured DNA nanostructures

Despite many studies on how geometry can be used to control the electronic properties of graphene, certain limitations to fabrication of designed graphene nanostructures exist. Here, we demonstrate controlled topographical replication of graphene by artificial deoxyribonucleic acid (DNA) nanostructures. Owing to the high degree of geometrical freedom of DNA nanostructures, we controlled the nanoscale topography of graphene. The topography of graphene replicated from DNA nanostructures showed enhanced thermal stability and revealed an interesting negative temperature coefficient of sheet resistivity when underlying DNA nanostructures were denatured at high temperatures.   

Seeded Growth of Highly Crystalline Molybdenum Disulphide Monolayers at Controlled Locations

Various approaches have been demonstrated for growth on insulating substrates of molybdenum disulphide (MoS$_{\mathrm{2}})$, but to date growth of isolated crystalline flakes has been only at random locations. We have developed a method to obtain MoS$_{\mathrm{2}}$ flakes in precisely defined locations. By patterning molybdenum source material that acted both as material feedstock and growth seed at predetermined areas across a wafer, we were able to grow isolated flakes of MoS$_{\mathrm{2}}$ at these locations with micrometre-scale resolution. These MoS$_{\mathrm{2}}$ flakes are predominantly of monolayer thickness and high material quality, as confirmed by atomic force microscopy, transmission electron microscopy, Raman and photoluminescence spectroscopy. Since the monolayer flakes are isolated and in predetermined locations, fabrication of transistor structures requires only a single lithographic step. Thus we are able to obtain multiple arrays of MoS$_{\mathrm{2}}$ transistors, that are highly crystalline and monolayer, making this method ideal for large scale production. Device measurements showed a carrier mobility and on/off ratio that exceeded 10 cm$^{\mathrm{2}}$V$^{\mathrm{-1}}$s$^{\mathrm{-1}}$ and 10$^{\mathrm{6}}$, respectively. This growth technique provides a path for in-depth physical analysis of monolayer MoS$_{\mathrm{2}}$ as well as fabrication of MoS$_{\mathrm{2}}$-based integrated circuits.   

Wafer-scale arrayed p-n junctions based on few-layer epitaxial GaTe

Two dimensional (2D) materials have showed appealing applications in electronics and optoelectronics. Gapless graphene presents ultra-broadband and fast photoresponse while the 2D semiconducting MoS2 and GaTe exhibit highly sensitive and tunable responsivity to the visible light. However, the device yield and its repeatability call for a further improvement of 2D materials to render large-scale uniformity. Here we report a layer-by-layer growth of the wafer-scale GaTe by molecular beam epitaxy. To develop the arrayed p-n junctions, the few-layer GaTe was grew on three-inch Si wafers. The resultant diodes reveal good rectifying characteristics and photoresponse with maximum photodetection responsivity of 2.74 A/W and photovoltaic external quantum efficiency up to 62{\%}. The photocurrent reaches saturation very fast within 22 $\mu $s and shows no sign of device degradation after 1.37 million cycles of operation. Most strikingly, such high performance has been achieved across the entire wafer, making the volume production of devices accessible. Finally, several photo-images was acquired by using these photodiodes with a reasonable contrast and resolution, demonstrating for the first time the potential for these 2D technology coming into the real life.