Session P17: 2D Materials: Assembly and Characterization

Towards Lego Snapping; Integration of Carbon Nanotubes and Few-Layer Graphene

Integration of semiconducting, conducting, and insulating nanomaterials into precisely aligned complicated systems is one of the main challenges to the ultimate size scaling of electronic devices, which is a key goal in nanoscience and nanotechnology. This integration could be made more effective through controlled alignment of the crystallographic lattices of the nanoscale components. Of the vast number of materials of atomically-thin materials, two of the sp2 bonded carbon structures, graphene and carbon nanotubes, are ideal candidates for this type of application since they are built from the same backbone carbon lattice. Here we report carbon nanotube and graphene hybrid nanostructures fabricated through their catalytic synthesis and etching. The growth formations we have investigated through various high-resolution microscopy techniques provide evidence of lego-snapped interfaces between nanotubes and graphene into device-relevant orientations. We will finish with a discussion of the various size and energy regimes relevant to these lego-snapped interfaces and their implications on developing these integrated formations.   

Two-dimensional square structures of CuO and Cu2O monolayer.

Among 2D crystals, monolayer (ML) oxides are interesting because of the coupling of quantum confinement to other degrees of freedom that are present in bulk materials. However, as most oxides are not layered structures, fabrication of 2D oxides has been limited. Current studies focus on either two-to-three atomic layers thick materials, such as the exfoliated perovskites, or supported films that are bonded to the substrate. Unsupported single-atom-thick oxides have not been reported. Here we report the fabrication of single-atom-thick copper oxide ML. Quantum mechanical calculations indicate that free-standing copper oxide MLs are stable wide-bandgap semiconductors with a variable chemical stoichiometry ranging from CuO to Cu2O at similar lattice constants. The stoichiometry variation changes the bandgap from indirect for CuO ML to direct for Cu2O ML, suggesting that the electronic and optical properties of ML copper oxides can be tuned by the oxygen content.   

Oxidation-derived two-dimensional MoO$_{\mathrm{3}}$/MoS$_{\mathrm{2}}$ heterostructures

In order to explore the efficient formation of MoO$_{\mathrm{3}}$/MoS$_{\mathrm{2}}$ heterostructures, we systematically investigated the course of surface oxidation of mechanically exfoliated single and few-layer MoS$_{\mathrm{2}}$ induced by oxygen plasma treatment using photoluminescence (PL) and Raman spectroscopy, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Raman and PL spectra served as sensitive indicators for defects generated by the plasma oxidation, showing Raman peak broadening and drastically reduced intensities for both Raman peaks and PL bands. XPS detected Mo$^{\mathrm{6+}}$ as the major Mo species in the oxidized samples, confirming the conversion of MoS$_{\mathrm{2}}$ into amorphous MoO$_{\mathrm{3}}$. The AFM studies also revealed that the thickness of MoS$_{\mathrm{2}}$ layers more than doubles when oxidized and that the vertical reaction from the top dominates with a negligible contribution from the lateral attack when combined with the Raman measurements. Our results show that the oxygen plasma treatment can be successfully used in generating atomically thin MoO$_{\mathrm{3}}$ or two-dimensional MoO$_{\mathrm{3}}$/MoS$_{\mathrm{2}}$ heterostructures that may be useful for future electronic and optoelectronic application.   

Microscopic theory of two-dimensional spatially-indirect-exciton condensates and exciton-polariton condensates

BEC of excitons and polaritons have drawn attention in recent years because of the demonstration of their ability to host macroscopic quantum phenomena and because of their promise for applications. We study the case of a system containing two TMD monolayers that are separated and surrounded by h-BN. Under appropriate conditions this system is expected to support a spatially indirect thermal equilibrium exciton condensate. We combine a microscopic mean-field calculation and a weakly interacting boson model to explore the bilayer exciton condensates phase diagram. By varying the layer separation and exciton density, we find a phase transition occurs between states containing one and two condensate flavors. We also use a microscopic time-dependent mean-field theory to address condensate collective mode spectra and quantum fluctuations. Next we study the case of exciton-polariton formed by strong coupling between quantum well excitons and confined photon modes when the system is placed in a vertical microcavity. We build a microscopic mean-field theory starting from electrons and holes, and account for their coupling to coherent light field. We compare our model with the normal weakly interacting boson model that starts from weakly interacting excitons that are coupled to photons.   

Preparation and Characterization of Large Area Monolayer Films of Pt Nanoparticles

Highly uniform monolayer thick coatings of Pt nanoparticles with areas as large as 20 cm$^{2}$ have been prepared by first self-assembling the desired Pt film at the interface between two immiscible liquids and then transferring the film to a glass substrate. The controlled addition of ethyl alcohol to a phase separated mixture of an aqueous colloidal solution of Pt nanoparticles and hexane allowed both monolayer and multilayer films to be prepared. Optical microscopy and UV-vis spectrophotometry measurements have been used to verify the large scale uniformity of the coatings while transmission electron and atomic force microscopy measurements confirmed that single and multilayer films can be prepared.   

The 2D Selfassembly of Benzimidazole and its Co-crystallization.

Benzimidazoles (BI) are organic molecules that form ferroelectric crystals. Key to their ferroelectric behavior are the switchable N$\cdot \cdot \cdot $HN type bonds and how they couple to the electron system of the molecules. We attempted to crystallize BI on various metal surfaces and studied them using STM. We observed that on Au and Ag, BI joins into zipper chains characteristic of its bulk structure that can pack into a continuous 2D layer. Because the dipole of BI lies in the direction of its switchable hydrogen bond, these zippers should in principle have reversible polarizations that point along the direction they run. BI's crystallization is reminiscent to how croconic acid (CA) crystallizes in 2D using O$\cdot \cdot \cdot $HO bonding, suggesting that these molecules may be able to co-crystallize through OH$\cdot \cdot \cdot $N bonds. This would present the opportunity to modify BI's properties, such as the energy needed to switch a hydrogen from a donor to acceptor site. When co-deposited, CA and BI successfully combine into a co-crystal formed by building blocks consisting of 2 CA and 2 BI molecules. These findings demonstrate the usefulness of using STM as a preliminary check to verify if two molecules are compatible with each other without having to attempt crystallization with multiple solvents and mixing methods.   

Correlating Structural and Electronic Degrees of Freedom in 2D Transition Metal Dichalcogenides

We have conducted a microscopic study of the interplay between structural and electronic degrees of freedom in two-dimensional (2D) transition metal dichalcogenide (TMD) monolayers, multilayers and heterostructures. Using the recently developed full field x-ray reflection interface microscopy with the photoluminescence microscopic probe capability at the Advanced Photon Source, we demonstrated the x-ray reflection imaging of a monolayer 2D material for the first time. The structural variation across an exfoliated WSe$_2$ monolayer is quantified by interlayer spacing relative to the crystal substrate and the smoothness of the layer. This structural information is correlated with the electronic properties of TMDs characterized by the {\it in-situ} photoluminescence measurements. This work is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-SC0012509. The use of Advanced Photon Source is supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357.   

Symmetry protected topological phases in a multi-band 2D electron gas.

Recently the observation of symmetry protected topological phases was reported in monolayer and bilayer graphenes in the $\nu =$ 0 quantum Hall state. Ground state in a multi-band Dirac systems such as ABA-trilayer graphene shows more complex phases than their mono- and bilayer counterparts. Tight-binding Hamiltonian in the absence of out-of-plane displacement field along with the mirror symmetry about the middle layer leads to a presence of non-interacting two bands. Relative shift of bands in the Landau Level energy spectrum map exhibits the existence of conductive counterpropagating phases with values $\sigma_{xx}_{\, }=$ 2$\alpha e^{2}/h$ where $\alpha \quad $is the spin-degeneracy$.$ In addition, out-of-plane displacement field plays a crucial role in mixing those bands leading to broken symmetries and polarizing charge carriers in only bottom or top layer. We will present our most recent studies on quantum Hall phase diagram of $\nu =$ 0 in ABA-trilayers.   

Thermoelectronic emission from monolayer graphene with temperature dependent work functions

For the first time we have derived an equation for the temperature (T) dependent work function (W(T)) that will be important for modeling thermoelectronic current density (J) and energy distribution of emitted electrons specially, from nano-materials. The equation containing terms up to fifth power of T gives a modified Richardson-Dushman (MRDE) equation that fits excellently well the experimental data of J vs T for suspended graphene. It provides a unique technique for accurate determination of W$_{\mathrm{0}}$, Fermi energy, E$_{\mathrm{F0}}$ at 0 K and surface density of charge carriers, n$_{\mathrm{s}}$ of graphene. The corresponding values obtained for suspended graphene are: W$_{\mathrm{0}} \quad =$ 4.42 \underline {}$+$ 0.01 eV, E$_{\mathrm{F0}} \quad =$ 0.166 \underline {}$+$ 0.002 eV; n$_{\mathrm{s}} \quad =$ 2.34x10$^{\mathrm{12}}$ cm$^{\mathrm{-2}}$. The model gives --ve thermal expansion coefficient of graphene (-8x10$^{\mathrm{-6}}$ /K) which has been experimentally confirmed. The equations are expected to hold for carbon nanotubes.   

Effect of dopant distribution on thermal conductivity of C\textunderscore (1-x) N\textunderscore x

The thermal conductivities of nanoscale nitrogen doped graphene (C\textunderscore (1-x) N\textunderscore x) with various nitrogen ratio and distribution is studied by performing nonequilibrium molecular dynamics. The thermal conductivity of randomly doped C\textunderscore (1-x) N\textunderscore x is found much smaller than that of the regular alloy when the dopant ratios are the same. Meanwhile, thermal conductivity of random alloys is dopant ratio sensitive while that of regular alloy is not. Interestingly, localization mode analysis indicates that the inequality of atoms under translation and inversion is responsible for the change of the thermal conductivity and a linear relationship between them is found. The results may provide a general guidance for phonon manipulation and thermal engineering in alloys.   

Doping and Thermal Conductivity Studies of CrSiTe3

CrSiTe3 is a layered material with a 2-dimensional crystal structure, and has recently become of more interest due to the possibility of using its ferromagnetic and semiconducting properties for spintronics applications. To further investigate the properties of CrSiTe3, we doped it with various transition elements on the Cr site in an attempt to tune and control the magnetism, as well as study changes in the thermal conductivity. We synthesized pure CrSiTe3 and doped samples through flux growth, producing plate-like bulk crystals. Crystal quality was checked by x-ray diffraction and energy dispersive spectroscopy, and then thermal conductivity and magnetization measurements were obtained on the doped materials to compare variations from the pristine CrSiTe3.   

Electronic and Mechanical Properties of Graphene-Germanium Interfaces Grown by Chemical Vapor Deposition

Epitaxial graphene grown directly on semiconducting Ge wafers holds potential for fundamental science and electronics applications. However, since the initial demonstration, little work has been done on the structural and electronic properties of this system. To gain insight into the interface between graphene and Ge, we performed ultra-high vacuum scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) along with Raman and X-ray photoelectron spectroscopy experiments to probe the atomic structure and chemistry at the interface. STS confirms stronger interfacial interaction on Ge(110), consistent with models of epitaxial growth. Raman spectroscopy shows that strain is highly prevalent after growth. Furthermore, the native strain modifies the atomic structure of the Germanium, inducing new and metastable Ge surface reconstructions following annealing. These reconstructions, in turn, modify both the electronic and mechanical properties of the graphene. Finally, graphene/Ge(001) represents the extremely strained case. Here graphene forces restructuring of the Ge surface into [107] facets. From this work, we see that the interaction between graphene and Ge is both dependent on the substrate crystallographic orientation and tunable.   

Optical activity and circular dichroism of plasmonic nanorod assemblies

Plasmonic circular dichroism (CD) has offered an efficient spectroscopy method for the electronic, chemical, and structural properties of different types of light active molecules in the subwavelength regime. Among the different chiral geometries of metal nanoparticles utilized by the plasmonic CD spectroscopy, gold nanorods (AuNRs) have shown strong CD signals in the visible frequency range. In this work, we theoretically study the CD signals of AuNR arrangements in order to mimic structures and chemical bonds of chiral biomolecules. In particular, our twisted three-AuNR geometries resemble a molecular structure of tartaric acid. This molecule played an important role in the discovery of chemical chirality. In our study, we show that the strength of CD signals changes dramatically by tuning the interparticle distances and angles. Since the CD signals are typically weak, we develop reliable computational approaches to calculate the plasmonic CD. Manipulating interparticle distances, size, and molecular bond angles result in full control over peak positions, handedness, and positive and negative bands which are observed in the CD spectra.   

Strong superchiral field in hot spots and its interaction with chiral molecules.

We have found that strong superchiral fields created by surface plasmon resonance exist in hot spots of nonchiral plasmonic structure, which showed a chiral density greater than that of circularly polarized light by hundreds of times. We have demonstrated a direct correlation between the chirality of the local field and the circular dichroism (CD) response at the plasmon resonance bands induced by chiral molecules in the hot spots. Our results reveal that the wavelength-dependent superchiral fields in the hot spots can play a crucial role in the determination of the plasmonic CD effect. This finding is in contrast to the currently accepted physical model in which the electromagnetic field intensity in hot spots is a key factor to determine the peak intensity of the plasmonic CD spectrum. Some related experimental phenomena have been explained by using our theoretical analysis.