Session B12: Undergraduate Research/SPS II

Electronic properties of two-dimensional MoS$_{\mathrm{2}}$ and its nanoribbons: a theoretical study

Recent advances in producing monolayer and bilayer molybdenum disulfide (MoS$_{\mathrm{2}})$ have generated considerable interest in its fundamental properties. Two-dimensional MoS$_{\mathrm{2}}$ has a variety of intriguing but unexplored electronic and optical properties that are dependent on the number of layers in the material. Here, we present calculations of the electronic properties of two-dimensional MoS$_{\mathrm{2}}$ and its nanoribbons based on a semi-empirical tight-binding model and ballistic quantum transport theory. Utilizing simulation results, we discuss the potential of two-dimensional MoS$_{\mathrm{2}}$ and its nanostructures for low-power electronic applications.   

Tuning charge density wave transition by introducing periodic strain patterns in thin 1T-TaS$_{\mathrm{2\thinspace }}$layers

Charge density waves (CDW) can significantly influence the electronic properties of materials. These correlated states form below a critical temperature which is controlled by the strength of the interactions between the conduction electrons and the crystal lattice. The CDW transition temperature is highly sensitive to lattice perturbations and distortions such as can be induced by strain or by reducing the sample thickness. We report on the effects of strain in addition to sample thickness on the CDW transition in thin 1T-TaS$_{\mathrm{2}}$~films. Periodically modulated strain fields are induced in the thin film samples by depositing them on an array of micron size Au pillars. The pillar structures are supported on a 300nm, SiO$_{\mathrm{2}}$~layer capping a highly doped Si backgate. We employ transport measurements to investigate the effect of the pillar-induced strain, sample thickness and array parameters on the transition temperature between commensurate and incommensurate CDWs.   

Chemical substrate transfer of topological insulator thin films for novel characterization and interfaces

Molecular beam epitaxy (MBE) is a widely used technique for synthesizing wafer scale samples of bismuth chalcogenide topological insulator (TI) thin films. Importantly, the deposition technique allows the fabrication of heterostructures wherein a TI is interfaced with symmetry breaking phases of matter such as ferromagnets, antiferromagnets and superconductors. However, the MBE of TI films on such substrates yields films of widely varying structural quality.1 An alternative approach is to grow a TI film with optimized structural quality on an appropriate substrate and then lift it off for transfer onto an arbitrary substrate of interest [Bansal \textit{et al}., Nano Lett. \textbf{14}, 1343 (2014)]. This method vastly expands the range of possible TI heterostructures.~We describe experiments wherein large area MBE-grown TI thin films are lifted off from sapphire substrates and then transferred onto other materials such as ferromagnetic insulators. These samples are characterized using electrical transport measurements, atomic force microscopy, and x-ray diffraction. We also describe plan view transmission electron microscopy (TEM) of the TI films by transfer to TEM grids, as well as attempts to fabricated TI thin films suspended over deep valleys in a substrate.   

Mechanical exfoliation of novel relativistic Mott insulators

Graphene, the first one atom thick crystal observed in nature, was successfully isolated by mechanical exfoliation, a process in which layers held together by van der Waals forces can be peeled apart with the help of scotch tape. Here we show the application of this method together with nanofabrication techniques to integrate exciting novel layered materials into an electronic device.   

Development and Application of Self-Oscillating LC Circuit Driven by a Topological Kondo Insulator in Extreme Conditions

Self-oscillating circuits are desirable for use in the study of quantum materials due to their high-precision but are challenging to execute in high-field, low-temperature environments. Recently, it has been shown that single crystals of the mixed-valence Kondo insulator SmB6 can exhibit negative differential resistance (NDR) in the characteristic V-I curve at low temperatures which could have useful applications in research of quantum materials. Here, we will present a study of the feasibility of using such crystals as elements in a self-oscillating circuit. We explore the bounds of the joule-heating-driven mechanisms with measurements down to 20mK temperatures and fields up to 20 Tesla. We will explore the tuning of crystal size and analyze the oscillating behavior with respect to temperature, field, size, and geometry, to develop a prototype for applying these crystals to generic studies of quantum materials.   

Analysis of Thermal Properties of Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ in the Range 10-300 K

The compounds PrOs$_4$Sb$_{12}$ and NdOs$_4$Sb$_{12}$ have attracted interest due to their exotic properties at low temperature. At low temperatures, the Neodymium compound becomes ferromagnetic , while the Praseodymium compound exhibits unconventional heavy-fermion superconductivity. The series of doped compounds Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ is being studied in order to understand the interaction between these effects. It has been shown that for particular concentrations of Nd and Pr, the phenomena of ferromagnetism and superconductivity are present simultaneously. In order to understand this system, it is necessary to characterize the normal-state behavior. The molar specific heat of Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ was measured in the range of 10-300 K, and thermodynamic parameters of the sample are extracted from the specific heat data, including the Debye Temperature, Einstein Temperature, and electronic specific heat coefficient. These provide information about the lattice softening, rattling effect, and electron correlation respectively. The evolution of these properties with respect to the Nd concentration, $x$, can then be determined.   

Synthesis and properties of new U3TiSb5-type compounds

Recently it was found that single crystals of Ce3TiSb5 exhibit a complex temperature/magnetic-field phase diagram with several metamagnetic transitions and a possible re-entrant disordered phase. In this presentation, I will discuss our efforts to synthesize and characterize other members of the ``3-1-5'' family of compounds. In particular, we synthesized single crystal of both Ce3ZrSb5 and Pr3TiSb5 using Sn flux. We find that Pr3TiSb5 exhibits similar magnetic transitions at high field as Ce3TiSb5.   

Exchange-coupled Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ nanoparticles for advanced magnetic hyperthermia

Iron oxide nanoparticles especially Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$, $\gamma $-Fe$_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$have been extensively studied for magnetic hyperthermia because of their tunable magnetic properties and stable suspension in superparamagnetic regime. However, their relatively low heating capacity hindered practical application. Recently, a large improvement in heating efficiency has been reported in exchange-coupled nanoparticles with exchange coupling between soft and hard magnetic phases. Here, we systematically studied the effect of core and shell size on the heating efficiency of the Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ core/shell nanoparticles. The nanoparticles were synthesized using thermal decomposition of organometallic precursors. Transmission electron microscopy (TEM) showed formation of spherical shaped Fe$_{\mathrm{3}}$O$_{\mathrm{4\thinspace }}$and Fe$_{\mathrm{3}}$O-/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ nanoparticles. Magnetic measurements showed high magnetization ($\cong $70 emu/g) and superparamagnetic behavior for the nanoparticles at room temperature. Magnetic hyperthermia results showed a large increase in specific absorption rate (SAR) for 8nm Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ compared to Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$ nanoparticles of the same size. The heating efficiency of the Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ with 1 nm CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ (shell) increased from 207 to 220 W/g (for 800 Oe) with increase in core size from 6 to 8 nm. The heating efficiency of the Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ with 2 nm CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ (shell) and core size of 8 nm increased from 220 to 460 W/g (for 800 Oe). These exchange-coupled Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/CoFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ core/shell nanoparticles can be a good candidate for advanced hyperthermia application.   

Selective Growth of PZT Nanowires on Si Substrates Using Glancing Angle Pulsed Laser Deposition

Thin films and nanostructures of the ferroelectric material Lead Zirconium Titanium Oxide (PZT) offer a multitude of applications in Piezotronics, and ferroelectric capacitor memories. While the growth of PZT thin films is well established, methodologies for the fabrication of vertically-aligned and spatially ordered PZT columns in nanoscale are not common. In this work an approach that uses a self-assembled nanoparticle template in a glancing angle pulsed laser deposition (GAPLD) process is presented. Lanthanum strontium manganite oxide (LSMO) was grown by laser ablation on a Si substrate masked by a monolayer of commercially available silica nanospheres (SNS) with diameter of 250nm self-assembled in a closed-pack hexagonal configuration (HCP) using Langmuir-Blodgett method. The HCP configuration of the mask will allows for the formation of LSMO islands on the crevices in between spheres, which will serve as seed layers for PZT growth. Scanning Electron Microscopy (SEM) was used to observe the grown PZT's morphology. Due to the ballistic shadowing effect introduced by the GAPLD, PZT columns in the form of hexagonal nanopillars evolved over the spatially ordered nanotemplate. Tunability of growth was achieved for certain PZT growth conditions. Morphological and structural properties of these structures were studied and showed a preferred orientation of growth of the (200) tetragonal/rhombohedral phase.   

Characteristics of Au nanowire arrays on GaAs photodetectors for optimal optical enhancement at near-infrared wavelengths

This research explores how the properties of Au plasmonic nanostructures on the plane of a GaAs semiconductor improve the total optical enhancement in GaAs photodetectors. All modeling was performed computationally to study these properties. Varying the electrode spacing, Au width, and Au thickness were shown to drastically affect the amount of enhancement in the GaAs. Peaks in enhancement were observed at specific Au widths and thicknesses resonant with the incident wavelength of 875 nm. The intensity of these peaks decayed as the widths and thicknesses increased. In addition, a simulation was run with a Ti adhesion layer between the Au and the GaAs. It was shown that as the Ti thickness increased, the optical enhancement in the semiconductor decreased. Increasing the thickness of the Au proved to shift the peak values in optical enhancement. It was also shown that electrode spacing and width of the structures played a more significant role than the period of the grating.   

Quantum entanglement between ballistic electrons and quantum-dot spins in carbon nanotubes

In this work we analyze the quantum entanglement between ballistic electrons and quantum-dot spins in semiconductor and metallic carbon nanotubes. The quantum-dot is modeled as a confined electron in a potential well. The interaction between the confined and the ballistic electrons is obtained through an effective interaction given by Coulombic and the quantum well potential. This interaction generates quantum correlation between the static and mobile electron spins. The electron dynamics is model by the Schrödinger equation for semiconductor carbon nanotubes and by the Dirac equation for metallic carbon nanotubes. The addition of mass term in the system in order to create a band gap and confinement of the electrons in metallic carbon nanotubes is discussed. Concurrence and quantum correlation parameter calculations are performed to obtain the level of spin quantum entanglement. We found that the resonance effect has an important impact on the quantum entanglement in the proposal systems.   

Colossal current driven conductance in artificial hybrid honeycomb system

The artificial magnetic honeycomb lattice has emerged as a new research arena to explore novel magnetic and electronic properties of materials. Flexibility in tuning the lattice parameters, as well as the materials' characteristics in the newly designed lattice of ultra-small bonds allows us to explore practical applications that are illusive in conventional magnetic materials. For instance, the moderate current driven colossal electrical conductance/resistance is still a challenge to the scientific community. Here we present new results on the observation of current driven colossal conductivity in hybrid artificial honeycomb lattice. We nano-fabricate the new system using a top down throughput approach, which results in macroscopic size sample with typical honeycomb bond dimension of 12 nm (length). 5 nm (width). Metallic layers of Sn (\textasciitilde 3.5 nm) and Nd (\textasciitilde 3 nm) are deposited in succession in ultra-high vacuum in order to create a clean lateral contact between the two metals. Detailed electronic and magnetic measurements at T $=$ 30 K on the new system reveal colossal change in conductivity on a moderate application of current (\textasciitilde 5 micro-A) in zero magnetic field. The current driven colossal conductance persists all the way to T $=$ 300 K, albeit weakly.   

Development of novel nanomaterials research project at a community college

At Hartnell College, we are developing an undergraduate research program in the synthesis ~and characterization of metallic nanoparticles and semiconducting quantum nano materials. Presently, we have synthesized silver nanoparticles using Turkevich method in which silver ions are reduced with sodium citrate. ~Due to recent reports on the prospect of bandgap engineering of cesium lead halide perovskite nanoparticles, we plan to synthesize aforementioned compounds and study their salient features. To characterize the resultant ~nanoparticles, material science techniques such as UV-visible absorption spectroscopy, scanning electron microscopy and atomic force microscopy would be used. ~Here, our synthetic and spectroscopic results are presented.   

Ab Initio Study of KCl and AgCl Clusters.

This paper presents a theoretical study of molecular clusters that examines the chemical and physical properties of small K$_{n}$Cl$_{n}$ and Ag$_{n}$Cl$_{n}$ clusters (n $=$ 2 - 24). Due to combinations of attractive and repulsive long-range forces, such clusters exhibit structural and dynamical behavior different from that of homogeneous clusters. The potentially important role of these molecular species in biochemical and medicinal processes is widely known. This work applies the hybrid ab initio methods to derive the different alkali-halide (M$_{n}$H$_{n})$ geometries. Of particular interest is the competition between hexagonal ring geometries and rock salt structures. Electronic energies, rotational constants, dipole moments, and vibrational frequencies for these geometries are calculated. Magic numbers for cluster stability are identified and are related to the property of cluster compactness. Mapping of the singlet, triplet, and quintet, potential energy surfaces is performed. Calculations were performed to examine the interactions of these clusters with some atoms and molecules of biological interest, including O, O2, and Fe. Potential design of new medicinal drugs is explored. We will also investigate model and material dependence of the results.