AVS2015 Session 2D+EM+NS+PS+SP+SS+TF-MoM: 2D Materials: Growth and Fabrication

Monday, October 19, 2015 8:20 AM in 212C

Monday Morning

Time Period MoM Sessions | Abstract Timeline | Topic 2D Sessions | Time Periods | Topics | AVS2015 Schedule

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8:20 AM 2D+EM+NS+PS+SP+SS+TF-MoM-1 Growth and FTIR Characterization of 2D Hexagonal Boron Nitride on Metal Substrates
Boris Feigelson, Victor Bermudez, Jennifer Hite, Zachary Robinson, Virginia Wheeler, Karthik Sridhara, Sandra Hernández (US Naval Research Laboratory)

Atomically thin two dimensional hexagonal boron nitride (2D h-BN) is one of the key materials in the development of new van der Waals heterostructures due to its outstanding properties including an atomically smooth surface, high thermal conductivity, high mechanical strength, chemical inertness and high electrical resistance. The development of 2D h-BN growth is still in the early stages and largely depends on rapid and accurate characterization of the grown monolayer or few layers h-BN films.

In this work, the IR-active out-of-plane vibrational mode of 2D h‑BN films grown in vertical reactor by atmospheric-pressure CVD on metal substrates (mainly Cu but also Ni) is exploited to identify 2D h‑BN directly on substrates and studied both computationally and experimentally.

Fourier transform grazing-incidence infrared reflection absorption spectroscopy (FT-IRRAS) data have been used to characterize monolayer and few-layer h-BN films directly on metal substrates. Two sub-bands of the A2u(LO) vibrational mode were, for the first time, found for thin 2D h-BN films in contact with Cu and Ni [1]. To unveil the nature of the discovered sub-bands, ab-initio calculations were performed and verified using 2D h-BN films grown on various Cu substrates with varying coverage and with individual crystallites of different shapes and size up to 4 mm. It was shown that the lower-energy A2u(LO)1 sub-band around 819 cm-1 is related to 2D h-BN coupled with Cu substrate, while the higher energy A2u(LO)2 sub-band around 824 cm-1 is related to decoupled (essentially free standing) 2D h-BN. These findings demonstrate not only a new and facile method for immediate 2D h-BN identification and characterization, but also a method that provides a simple means to characterize the degree of coupling between 2D h-BN and the substrate. This approach also provides an opportunity to determine which growth conditions lead to the absorption of foreign species on the substrate prior to the h-BN deposition and which conditions can prevent the formation of the interfacial layer between h-BN and the substrate. Such interfacial layers, like oxidized Cu, were shown to result in easily-recognizable shifts in the A2u(LO) peak. The degree to which the interaction of the h BN layer with the substrate is uniform and homogenous can also be assessed easily by examining the width and fine structure of the A2u(LO) band. The developed approach can also be used to study growth and formation of h-BN/graphene and other 2D heterostructures.

References

1. B. N. Feigelson, V. M. Bermudez, J. K. Hite, Z. R. Robinson, V. D. Wheeler, K. Sridhara, and S. C. Hernandez, Nanoscale 7, 3694 (2015)

8:40 AM 2D+EM+NS+PS+SP+SS+TF-MoM-2 Effect of Surface Termination on the Growth of Graphene on Cu Single Crystal Substrates
Tyler Mowll, EngWen Ong (University at Albany-SUNY); Parul Tyagi (GLOBALFOUNDRIES); Zachary Robinson (College at Brockport-SUNY); Carl Ventrice, Jr. (SUNY Polytechnic Institute)
The most common technique for synthesizing single-layer graphene films with large lateral dimensions is chemical vapor deposition (CVD) on Cu foil substrates. The primary reasons for choosing Cu substrates are the extremely low solubility of carbon in Cu, which allows a self-limited growth of graphene, and the relatively low cost of the Cu foil substrates. However, the transport properties of the CVD grown graphene films are typically a couple of orders of magnitude lower than for graphene flakes mechanically exfoliated from graphite. One of the reasons for the reduction in transport properties is the presence of crystalline defects in the CVD grown films. These structural defects arise in part from the multidomain structure of the Cu films. In order to achieve a better understanding of the influence of the surface termination of the Cu substrate on the crystallization of graphene during the CVD growth process, a systematic study of graphene growth on Cu(100), Cu(110), and Cu(111) crystals has been performed. The growth process is performed in an ultra-high vacuum (UHV) chamber that has been modified to perform CVD growth at pressures as high as 100 mTorr. The precursor gas used is ethylene. This growth procedure allows for the preparation of the clean surfaces in UHV, growth under typical CVD conditions, and characterization of the surface structure in UHV, without exposing the sample to atmospheric contaminants. Our results indicate that the Cu(111) surface has the lowest catalytic activity of the three surfaces for the decomposition of ethylene. In fact, the decomposition rate is so low that graphene growth is suppressed because of the sublimation of Cu at the elevated temperatures used to grow the graphene. By using an Ar overpressure, it was found that graphene could be grown on that surface. The surface symmetry of the Cu substrate has a strong influence on the rotational alignment of the graphene grains as they nucleate on each surface. For Cu(111), single-domain graphene growth can be achieved for ethylene pressures of 5 mTorr or less. For both Cu(100) and Cu(110), a minimum of two graphene domains is always observed.
9:00 AM 2D+EM+NS+PS+SP+SS+TF-MoM-3 Thermally Annealed and Electropolished Cu Substrates for CVD Growth of 2D Materials: Graphene, h-BN and MoS2
Karthik Sridhara (Texas A&M University); Boris Feigelson, Jennifer Hite (US Naval Research Laboratory); Anindya Nath (George Mason University); Michael Fuhrer (Monash University, Australia); D.Kurt Gaskill (US Naval Research Laboratory); Homero Castaneda, Luke Nyakiti (Texas A&M University)

The growth of two dimensional (2D) materials such as graphene, hexagonal boron nitride (h-BN) and molybdenum disulphide (MoS2) have been demonstrated by chemical vapor deposition (CVD) on polycrystalline catalytic copper substrates. These Cu foil substrates (25 µm thick) are produced by metallurgical rolling leading to the formation of irregular ridges on the foil surface along with a film of native oxide on the surface. These processing artifacts are a limiting factor for controlled and reproducible large area (several cm2) growth of 2D materials. Greater control of growth can be achieved by controlling the density of nucleation sites and improving the catalytic activity of Cu by removing the Cu native oxide on the surface. Previous attempts to pre-treat the Cu substrate by using wet chemistry or thermal annealing to control growth has been weakly addressed.

In this work, electropolishing combined with prior thermal annealing at 1030°C for 5 hrs under H2 is used to control the degree of roughness of cold rolled polycrystalline Cu foils, and subsequently, to explore the influence of electropolishing on the growth of 2D materials: graphene, h-BN and MoS2. Electropolishing dissolves a thin surface layer of Cu, which contains surface defects and contaminants. This helps in decreasing the density of spontaneous nucleation sites by producing a morphologically uniform and contaminant-free surface. Secondary effects, etch pits which are ascribed to O2 bubbling at random nucleation sites on Cu surface, are mitigated by using additives, such as acetic acid and ethylene glycol, in the H3PO4 electrolyte. Thermal annealing and electropolishing results in this work reveal that a roughness of ~1.2 nm (Rq) can be achieved as measured by Atomic Force Microscope (AFM) along with a greatly planarized Cu foil. AFM will also be used to establish the Cu substrate morphology and its relationship to the growth of 2D materials. Fourier Transform Infrared, and Raman spectroscopy will be used to confirm the existence of the 2D material. Preliminary growth studies of h-BN on these high quality Cu substrates demonstrate improved growth, as assessed by the metrics of size and count of h-BN crystals from Scanning Electron Microscopy (SEM) micrographs [1]. This work will demonstrate that thermal annealing followed by electropolishing leads to optimization of Cu foil surface resulting in the larger crystal size and a reduction in nucleation sites that induce 2D material crystal growth [1].

[1] K. Sridhara. “Growth of hexagonal boron nitride on electrochemically prepared polycrystalline Cu substrates.” M.S. Thesis, University of Maryland, College Park, MD, 2014.

9:20 AM 2D+EM+NS+PS+SP+SS+TF-MoM-4 In Situ Optical Diagnostics During Molybdenum Disulfide Chemical Vapor Deposition
Berc Kalanyan, James Maslar, William Kimes, Brent Sperling (National Institute of Standards and Technology (NIST)); Robert Tieckelmann, Tommaso Orzali (SEMATECH); Ryan Beams, Stephan Stranick, Albert Davydov (National Institute of Standards and Technology (NIST))

Two dimensional (2D), layered transition-metal dichalcogenides (TMDs), e.g., MoS2, are of increasing interest for next-generation nanoelectronic and optoelectronic devices. These materials have thickness dependent optical and electrical properties that make them suitable for a variety of applications including integrated circuits. For many applications, high volume manufacturing (HVM) of devices based on TMDs will require deposition techniques that are capable of reproducibly growing wafer-scale, 2D TMD films with monolayer control. To date, such a capability has not been widely demonstrated with typical TMD deposition processes.

This work aims to identify promising chemistries for HVM TMD chemical vapor deposition (CVD) processes. We focus on MoS2 CVD using a variety of precursors (including organometallics, elemental sulfur, and organosulfur compounds) in a research grade single-wafer deposition system equipped with in situ optical diagnostics. The precursor flux is measured using optical mass flow meters installed on the delivery lines while deposition chemistry is characterized in the reactor volume above the deposition surface using in situ Fourier transform infrared (FR-IR) spectroscopy. As-deposited and annealed films are characterized with ex situ techniques, including Raman and photoluminescence spectroscopy, scanning and transmission electron microscopy, and X-ray photoelectron spectroscopy.

Stoichiometric MoS2 films have been prepared from (η5-ethylcyclopentadienyl)-dicarbonylnitrosyl molybdenum and elemental sulfur. As-grown films are smooth and continuous with major MoS2 Raman modes present. Film thickness scales approximately with Mo precursor exposure time and few-layer films can be produced using pulsed injection mode. Furthermore, optical in situ diagnostics allow us to relate metal precursor flux to film crystallinity and facilitate the study of precursor decomposition in the thermal boundary layer.

9:40 AM 2D+EM+NS+PS+SP+SS+TF-MoM-5 Controlled Interfaces in 2D Materials
Arend van der Zande (University of Illinois at Urbana Champaign)

Interfaces are ubiquitous in material science and technologies. For example, grain boundaries often dominate the mechanical and electrical properties in crystalline materials, while interfaces between dissimilar materials form the fundamental building blocks to diverse technologies, such as building electrical contacts in transistors and PN diodes in solar cells. Interfaces become even more important in 2D materials such as graphene and transition metal dichalcogenides, where the lack of dangling bonds enables material stability down to a single monolayer. In this entirely surface-dominated limit, the usual rules governing 3D interface devices, such as depletion regions, break down.

In this talk, we will discuss our work on engineering in- and out-of-plane 2D materials interfaces. We will first examine the structure of atomically-thin membranes and the impact of defects such as grain boundaries on the mechanical, optical, and electronic properties. We fabricate out-of-plane interfaces by stacking 2D materials to form heterostructures, which we utilize to tailor the bandgap in 2D materials and build new optoelectronic devices such as tunable photodiodes. Looking to the future, the rapidly expanding family of 2D materials with a diverse set of electronic properties provide a promising palette for discovering emergent phenomena and a motivation for developing overarching design principles for understanding and controlling interfaces in 2D.

10:20 AM BREAK
10:40 AM 2D+EM+NS+PS+SP+SS+TF-MoM-8 Obtaining Clean Suspended CVD Graphene: Comparative Examination of Few Transfer and Cleaning Protocols
Alexander Yulaev (National Institute of Standards and Technology (NIST), University of Maryland (UMD)); Guangjun Cheng, Angela Hight Walker (National Institute of Standards and Technology (NIST)); Marina Leite (University of Maryland (UMD)); Andrei Kolmakov (NIST)

Clean suspended graphene is used as supporting media in TEM, filtering membranes, and as electron transparent windows in ambient pressure electron spectroscopy and microscopy. CVD grown graphene is the most popular material for these applications due to its large-scale and high yield production. Multiple approaches such as sacrificial layer based methods [1] and direct transfer method on perforated carbon mesh by IPA droplet [2] have been implemented to transport graphene from copper or nickel foil onto a target substrate. However, the cleanness of the suspended graphene remains to be an issue, and controversial results on lateral size of atomically clean graphene domains have been reported [2-5]. We conduct the comparative analysis of the most widely-used CVD graphene transfer and cleaning protocols. In particular, using extreme surface sensitivity of low energy SEM, we compare the standard PMMA based approach with direct graphene transfer method. We also propose alternative graphene transfer protocol which is based on employment of polycyclic aromatic hydrocarbon (PAH) as a sacrificial layer. The advantage of PAH method over others consists in facile sublimation of sacrificial layer upon heating PAH material within moderate temperature range of 100-150 oC. All three methods of graphene transfer were compared under the same conditions followed by similar graphene cleaning procedures by platinum catalysis [4] and activated carbon adsorption [5]. Both SEM and TEM study revealed the superiority of PAH method to achieve cleaner suspended CVD graphene. We envision that novel approach of graphene transfer can be employed under conditions when exposure of the sample to moisture is prohibited such as in battery research.

[1] “Transfer of CVD-Grown Monolayer Graphene onto Arbitrary Substrates”, Ji Won Suk et al., ACS Nano, 2011, 5 (9), pp. 6916.

[2] “A direct transfer of layer-area graphene”, William Regan et al., Appl. Phys. Lett., 2010, 96, 113102.

[3] “Low-energy electron holographic imaging of gold nanorods supported by ultraclean graphene”, Jean-Nicolas Longchamp et al., Ultramicroscopy 145 (2014) 80.

[4] “Ultraclean freestanding graphene by platinum-metal catalysis”, Jean-Nicolas Longchamp et al., J. Vac. Sci. Technol. B 31, 020605 (2013).

[5] “Dry-cleaning of graphene”, Gerardo Algara-Siller et al., Applied Physics Letters 104, 153115 (2014).

11:00 AM 2D+EM+NS+PS+SP+SS+TF-MoM-9 Low-Energy Electron Microscopy of Transition Metal Dichalcogenides Prepared by Various Methods
Sergio de la Barrera, Siddarth Satpathy, Randall Feenstra (Carnegie Mellon University); Sanfeng Wu, Xiaodong Xu (University of Washington); Suresh Vishwanath, Xinyu Liu, Jacek Furdyna, Debdeep Jena, Huili Xing (University of Notre Dame); Yu-Chuan Lin, Sarah Eichfeld, Joshua Robinson (Pennsylvania State University); Patrick Mende (Carnegie Mellon University)

Recent work on two-dimensional materials has focused on transition metal dichalcogenides (TMDs), owing to their semiconducting behavior. Characterizing as-grown TMDs is crucial in improving the understanding of the effects of growth conditions, and ultimately improving material quality. Low-energy electron microscopy (LEEM) is a powerful tool for this purpose, providing real-space images with ~10 nm spatial resolution as well as selected-area low-energy electron diffraction (µLEED) of local crystal orientation at length scales down to ~ 1 µm. Additionally, by varying the incident electron beam energy, low-energy electron reflectivity (LEER) spectra are extracted.

In this work, comparative LEEM results are presented from three TMD materials: MoS2 prepared by exfoliation (onto Si), MoSe2 grown by molecular beam epitaxy (MBE) (on epitaxial graphene), and WSe2 grown by chemical vapor deposition (CVD) (also on epitaxial graphene). It is found that for TMDs generally, the LEER spectra do not exhibit the oscillatory behavior (in the 0 – 6 eV range) that is seen for both graphene and hexagonal boron nitride (h-BN) for various numbers of monolayers (MLs). This lack of oscillatory behavior is interpreted as being due to the weak coupling of the interlayer states localized in between the MLs, which is itself a result of the relatively large out-of-plane lattice parameter. Nevertheless, additional “band structure” features in the LEER spectra permit clear identification of the TMD materials relative to the substrates. The exfoliated flakes are seen to extend over many 10’s of mm, the MBE-grown MoSe2 forms a nearly continuous film, and the CVD-grown WSe2 forms triangular islands several mm in extent. µLEED studies of the MBE-grown MoSe2 and CVD-grown WSe2 reveal preferential orientation with the underlying graphene substrates.

The reduced work functions of the TMD materials relative to the underlying substrate are clearly evident in the onset voltages for the LEER spectra (i.e. the onset shifts in accordance with the local work function of the surface). Most significantly, for the WSe2 islands, a predominant “tail” is observed in this onset, extending about 5V below the usual onset location. This tail is tentatively interpreted as arising from charging of the islands, perhaps due to polar termination at the edges of the TMD islands. Comparison of the data with simulated LEER spectra will be presented, as a test of this model for edge charge of the islands.

Work supported by the Center for Low Energy Systems Technology (LEAST), one of six SRC STARnet Centers sponsored by MARCO and DARPA, and by NSF-EFRI-1433496.

11:20 AM 2D+EM+NS+PS+SP+SS+TF-MoM-10 Atomically-Thin 2D Layers of Group IV Semiconductors
Joshua Goldberger (The Ohio State University)

Similar to how carbon networks can be sculpted into low-dimensional allotropes such as fullerenes, nanotubes, and graphene with fundamentally different properties, it is possible to create similar “allotropes” of Ge or Sn with unique optoelectronic properties as well. Here, we will describe our recent success in the creation of hydrogen and organic-terminated group 14 graphane analogues, from the topochemical deintercalation of precursor Zintl phases, such as CaGe2. We will discuss how the optical, electronic, and thermal properties of these materials can be systematically controlled by substituting either the surface ligand or via alloying with other Group 14 elements. Additionally, we have also developed an epitopotaxial approach for integrating precise thicknesses of Germanane layers onto Ge wafers that combines the epitaxial deposition of CaGe2 precursor phases with the topotactic interconversion into the 2D material. Finally, we will describe our recent efforts on the synthesis and crystal structures of Sn-containing graphane alloys in order to access novel topological phenomena predicted to occur in these graphanes.

Time Period MoM Sessions | Abstract Timeline | Topic 2D Sessions | Time Periods | Topics | AVS2015 Schedule