Session K66: Surface and Interface Science

Investigation of the surface morphology of 4H-SiC Implanted with Low-Energy H and He Ions

Low-energy ions have the potential to be used for modifying materials on the nanoscale.  We have investigated low-energy ion-implanted 4H-SiC, employing x-ray and neutron reflectivity to obtain depth and density while using real-space imaging to examine the effect of implantation on the surface morphology. The SiC was implanted with H and He ions at <1keV using a standard ion gun.  Reflectivity measurements show a surface layer having a reduced density that is 60-65% of the density of SiC. The depth of this lower-density layer increases with sequential implantations.  The combination of x-ray and neutron reflectivity reveal that H remains embedded in the SiC.  Imaging techniques, Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), and Optical Profilometry (OP), show topographic surface defects. The number density for the area of these defects follows a power-law over several decades and confirms a surface density that is 60-80% of SiC, which is in agreement with the reflectivity results.   

Nuclear Spin Conversion of Molecularly Chemisorbed H2 on Pd(210)

It has been known that physisorbed hydrogen undergoes nuclear spin conversion (NSC) in the order of minutes on non-magnetic metal surfaces such as Ag(111)[1]. This has been described by the virtual surface-molecule electron transfer and the hyperfine Fermi contact interaction within second order perturbation theory. Recent experiments have shown that on Cu(510)[2] and Pd(210)[3] surfaces, H₂ is in the so-called molecularly chemisorbed state and behaves as a 2-dimensional quantum rotor, while the NSC rate is 1 order magnitude faster than on flat surfaces.

In this work, we investigate the NSC of the molecularly chemisorbed H₂ on Pd(210). The nuclear spin transition probability is evaluated by means of second order perturbation theory using the parameters derived from first principles calculations[4] . Our results show that the NSC time of H₂/Pd(210) is around 2 seconds, in agreement with experiment. Such a fast NSC rate can be attributed to a number of factors such as the substrate work function, the density of states at the Fermi level the electronic molecule-surface coupling potential. Both the work function and electronic density of states at the Fermi level of Pd(210) are larger compared with Ag(111), resulting in the increased NSC probability. In addition, the increased molecule-surface interaction in the molecularly chemisorbed state enhances the matrix element describing the virtual electron transfer. This effect further increases the NSC probability and rate.   

Theory of near-field electrostatic effects in supported and decorated 2D materials

Far-field electrostatic effecfs at interfaces are known to strongly modulate surface workfunctions through the areal dipole of the interface electronic density. However, for thin-films and integrated two-dimensional materials, the near-field effects resulting from higher moments of the interface electronic density may also impact optoelectronic properties. 

In this work, we derive a theory of such effects, and validate it through first-principles calculations. We develop a classical electrostatic model of the 2D materials / substrate interactions beyond the multipole expansion of the potential. Our theory captures the magnitude, modulations, and decay lengths of near-field effects using simple materials descriptors (i.e. atomic structures and charge density multipoles) [1].  and energy scales of the higher moments effects.

We discuss the implications of our theory for device functionality  showing how such effects allow to tune the momentum-dependent polarizability of a 2D metal as a function of its proximity to a metallic substrate [2], and can lead to bandgap opening for graphene  decorated by a phthalocyanine monolayer [1].   

Magnetic and Interfacial Characteristics of CMOS Compatible Sputtered Topological Insulator/Ferromagnet Heterostructures

The growth and magnetic properties of sputtered topological insulator (TI)/ferromagnet (FM) heterostructures are presented. Magnetron sputtering produced TI, Bi₂Te₃ (BT) with varying crystalline order: ranging from largely amorphous to highly c-axis-oriented. A giant enhancement in Gilbert damping in TI/FM heterostructures was observed for c-axis oriented BT. Heterostructures of BT/Ni₈₀Fe₂₀ (Py) produced interface phases of Ni-intercalated BT (Bi₂Te₃:Ni). The Bi₂Te₃:Ni interface layer was found to support a topologically nontrivial, novel AFM phase [1]. The AFM exchange interaction strength was found to decrease dramatically for the highly amorphous BT sample [2], as evidenced by the reduction in exchange bias in hysteresis loop measurements, from 8 mT to 1 mT at 6 K, and reduction in Neél temperature from 63 K to 20 K. Further, we performed spin-torque ferromagnetic resonance experiments in the BT/Py heterostructures taking into account the distinct interface layers, showing a larger spin-Hall angle in amorphous TIs compared with the c-axis oriented TIs by ~25%, which aligns with current understanding of sputtered TIs in literature. These results pave the way for the integration of sputtered TIs in industrial CMOS devices.   

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To calculate the conductivity of a material having full knowledge of its composition is a reasonably simple task. To do the same in reverse, i.e., to find information about the composition of a device from its conductivity response alone, is very challenging and even more so in the presence of disorder. An inversion methodology capable of  decoding the information contained in the conductivity response of disordered structures has been recently proposed but despite claims of generality and robustness, the method has only been used with 2D systems possessing relatively simple electronic structures. We put these claims to the test and generalise the inversion method to the case of monolayer MoS₂, a material whose electronic structure is far more complex and elaborate.

Starting from the spectral function that describes the DC conductivity of a disordered sample of a single layered MoS₂ containing a small concentration of randomly dispersed vacancies, we are able to invert the signal and find the exact composition of defects with an impressive degree of accuracy. Remarkably, equally accurate results are obtained with the optical conductivity. This is indicative of a methodology that is indeed suitable to extract composition information from different 2D materials, regardless of their electronic structure complexity. Calculated conductivity results were used as a proxy for their experimental counterpart and were obtained with an efficient quantum transport code (KITE) based on a real-space multi-orbital tight-binding model with parameters generated by density functional theory.    

Mechanisms of the Resistivity Size Effect in Metallic Thin Films Computed Using Realistic Tight-Binding Models and the Kernel-Polynomial Method

The resistivity size effect is an important limitation in device interconnect technology. Specifically, as the dimensions of interconnect wires approach ~10nm scales, increased resistivity becomes a fundamental limitation to device performance. While it has been understood that electron scattering from surfaces and grain-boundaries are responsible, dominant sources of scattering remain somewhat unclear. Computational approaches can play an important role in elucidating the resistivity size effect.

In this contribution, an approach based on realistic tight-binding models fit carefully to Density-Functional Theory (DFT) results is coupled with an efficient numerical approach based on the Kernel-Polynomial Method (KPM) is used to compute resistivity in Ruthenium thin films with the addition of steps and static phonon disorder. It is shown that single-crystalline films, which have been realized experimentally, can exhibit resistivity values comparable to bulk values, and that surface steps have a minimal impact on resistivity. The potential role of electron-phonon scattering at surfaces is also discussed.   

Coherent Electronic Band Structure of TiTe2/TiSe2 Moiré Bilayer

Bilayers created by stacking single layers from different van-der-Waals materials provide a promising platform for creating novel properties, despite a weak interlayer interaction expected from the van-der-Waals bonding. The interlayer interaction is expected to be even weaker when the two single layers are incommensurate, giving rise a moire modulation. Here, we report a detailed study of such a bilayer system made of a single layer of TiTe₂ atop a single layer of TiSe₂, both prepared by molecular beam epitaxy. The two layers assume the same crystallographic orientation, but he lattice mismatch creates a hexagonal moire superlattice with a large lattice constant of 56.6 Å. Angle-resolved photoemission measurements reveal sharp emergent bands that are distinct from the bands of the single layers alone. First-principles calculations demonstrate that the emergent bands arise from the coherent coupling of the wave functions over the different parts of the stacking variations in the moiré unit cell. This work illustrates a new route for engineering the band structure of van-der-Waals materials through layer stacking and demonstrates an effective methodology for theoretical analysis of the electronic structure of such systems.   

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Using a q+ atomic force microscopy at low temperature, a sexiphenyl molecule is slid across an atomically flat Ag(111) surface along the direction parallel to its molecular axis and sideways to the axis. Despite identical contact area and underlying surface geometry, the lateral force required to move the molecule in the direction parallel to its molecular axis is found to be about half of that required to move it sideways. The origin of the lateral force anisotropy observed here is traced to the one-dimensional shape of the molecule, which is further confirmed by molecular dynamics simulations. We also demonstrate that scanning tunneling microscopy can be used to determine the comparative lateral force qualitatively. The observed one-dimensional lateral force anisotropy may have important implications in atomic scale frictional phenomena on materials surfaces.   

Subatomic surface roughness in nanometer scale Au/MoS2 films

We have discovered that many metals exhibit electronic growth modes when deposited on the surface of certain dichalcogenides. In these systems, discrete structures spontaneously form with length scales corresponding to integer multiples of the metal's Fermi surface wavelength. Density functional theory (DFT) calculations can be predictive of this behavior. When electronic growth occurs, DFT calculations predict a highly quantized density of states, whereas in metals that do not display electronic growth modes the density of states is nearly bulk-like. In the Au/MoS₂ system, the electronic growth is especially strong, with discrete length scales up to 7 nm. Stable film heights are found with a height interval of 1.8 nm, which corresponds to the Au(111) Fermi surface wavelength. The 7.1 nm film height is especially stable, and marks a termination point for electronic growth. For coverages exceeding this height, the systems transofrms to epitaxial growth to form a nearly perfect Au(111) surface. The surface roughness is less than 50 pm over areas spanning several square microns, with no signs of grain boundaris, and the classic herringbone patterns can be seen in our scanning tunneling microscopy measurements. This was quite surprising given the lattice mismatch betwen Au and MoS₂ exceeds 8%. We believe this phenomena is arises from the stability of the 7.1 nm thickness arising from electronic growth. This serves as a platform in which the Au film assumes a bulk-like bonding arrangement so that further deposition occurs as if the substrate was akin to single crystal Au(111). We are currently exploring the potential for these films to serve as ultra-flat substrates for the study of self-assembled monolayers.    

Tunable Ergodicity of Adsorbates on Graphene with Substrate Potential Energy Landscape Engineering

The motion of surface adsorbates over substrates remains of fundamental importance in condensed matter physics with applications in nanoscale molecular devices. Moire heterostructures, consisting of twisted layers of periodic two-dimensional materials, provide a promising route for modifying the substrate and device tunability through moire twist angle engineering. As of yet, the effects of moire substrate engineering on the diffusion of surface adsorbates has not been studied. Here, we model the diffusion of surface adsorbates on a graphene-BN moire heterostructure of varying periodicities using a continuous time Markov chain. We find that the potential energy landscape of the moire lattice induces a weak ergodicity breaking, changing the stochastic motion of adsorbates from Brownian motion to continuous time random walk.  Using a deep neural network, we demonstrate the ability to detect this transition using only a few hundreds of short single molecule tracks, which makes the analysis applicable under experimental constraints. This study illustrates a new mode for controlling the stochastic motion of molecules in nanoscale devices.   

Transport in Layered Ternary Transition-metal Tellurides Ta2M3Te5 (M = Pd, Ni)

Two-dimensional topological insulator, also known as quantum spin Hall (QSH) insulator, exhibits novel one dimensional helical edge states at the boundaries. The robust conducting edge with forbidden back scattering is the consequence of the nontrivial topology of the insulating bulk band protected by time-reversal symmetry. Therefore, QSH insulators have great promise for potential application in spintronics. However, QSH state has been confirmed in limited systems. Recently, it was proposed that Ta₂M₃Te₅ are potential candidates for realizing the QSH effect, which is further tuned by strain. Also, the angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy measurements show a band gap at the Fermi level and topological edge states inside the gap of Ta₂Pd₃Te₅. Here, we report the transport study of Ta₂M₃Te₅ nanoflakes. The materials were synthesized by chemical vapor transport and characterized by EDS, XRD, TEM, and Raman. Nanodevices were fabricated for the transport measurement. The temperature-dependent resistivity of Ta₂Ni₃Te₅ shows the semiconductor properties with a small bandgap. The Hall effect and magnetoresistance have also been measured. The transport results show that Ta₂M₃Te₅ could be potential platforms for exploring QSH states.   

Exploring Substitution Doping of Fe in WTe2 Using Scanning Tunneling Microscopy and Spectroscopy (STM/S)

Transition Metal Dichalcogenides (TMDs) have been shown to intrinsically possess wide range of material properties from metallic to insulating and a variety of topological phenomena. Hence, TMD materials are of importance in electronics and photonics. In general, TMDs in their intrinsic state are nonmagnetic. The magnetism is expected to be introduced by doping with magnetic elements.

Here, we present preliminary results from scanning tunneling microscopy and spectroscopy (STM/S) on Fe doped WTe₂ (Fe_xW_1-xTe₂, x = 2.6%). At this doping level, Fe is revealed to be at substitution rather than intercalation sites through experimental STM/S study and by the density functional theory (DFT) calculations. The total doping concentration observed with STM/S agrees with the results obtained from the energy dispersive X-ray spectroscopy (EDS). Furthermore, STM/S data shows that the Fe substitution are equally probable at the two possible W sites in the unit cell. The Fe dopants provide electron doping as determined by Hall measurements as well as dI/dV spectra in STM/S measurements.   

Size and Print Path Effects on Mechanical Properties of Materials Extrusion 3D Printed Plastics

Optimization of mechanical properties through print conditions of thermoplastics by Material Extrusion (MatEx) has been commonly utilized, but this optimization tends to neglect dependence of properties on the local process history. Here, we systematically investigate effect of part size with standard tensile test specimens (ASTM D638 Type IV or V), print orientation and printing multiplicity on mechanical properties. Both polycarbonate (PC) and polypropylene (PP) were printed to understand crystalline vs. glassy plastics. For both printed PP and PC, elastic modulus was independent of specimen type as well as multiplicity, and weakly dependent on the print orientations. However, failure of PC was strongly dependent on the size of specimen with the smaller Type V exhibiting ductile failure while brittle failure was only observed with the Type IV specimens. Through examination of single orientation prints, the ductility of the Type V specimens is associated with the intralayer cooling time between adjacent extruded roads. In contrast, all of PP specimens exhibited ductile failure. These differences between PP and PC can be attributed to the mechanism of solidification, which changes the requirements for a strong interface as well as the temperature dependence of the viscosity.