Each student will undertake an individual research project, that is chosen based on the student’s academic background and interests to permit them the opportunity not only to contribute effectively to the research, but also to develop individually as young scientists. The students will engage in research projects that are inspired by or expand upon on-going investigations in nanomanufacturing, including projects on thin film growth of 2D chalcogenides and complex oxides, modeling, electrical and electromechanical characterization, and device integration. Brief project descriptions of possible research projects are included below.
Top-down and bottom-up nanomanufacturing
Prof. Susan Trolier-McKinstry (MatSE/EE): Scalable Nanomanufacturing of Piezoelectric Films
Piezoelectric materials convert between electrical and mechanical energies, and are useful in sensors, actuators and energy harvesting applications. Students will work on the development of deposition and patterning processes for high efficiency piezoelectric thin films for applications including filters for cell phones, miniaturized medical ultrasound transducers systems, and local power sources for the internet of things. It is hypothesized that control of defect chemistry will enable films with superior lifetime, and that the defect chemistry can be controlled through processing. This will be tested by conducting impedance, deep level transient spectroscopy, thermally stimulated depolarization current measurements, and highly accelerated lifetime measurements for samples processed in different ways.
Prof. Suzanne Mohney (MATSE/EE): Atomic Layer Deposition of Metals
Atomic layer deposition (ALD) is an ideal technique for creating a wide variety of layered materials with nanoscale precision. However, it is sometimes hindered by difficulties that arise during initial nucleation of the film. In this project, we will address the challenging problem of nucleation of metal layers during ALD on oxides or other dielectrics. To do so, we will test the hypothesis that nucleation can be enhanced by altering the functional groups on the substrate surface, including the number and types of hydroxyl groups, using ex situ and in situ chemical surface treatments. The REU student will also receive training on characterization of metal films both structurally and electrically, including through scanning electron microscopy, atomic force microscopy, and four-point probe measurements.
Prof. Brian Foley (Mech E): Measuring Thermal Transport across Chalcogenide-based Heterointerfaces
Continued interest in the use of 2D chalcogenides for electronic applications raises important questions related to eventual thermal management strategies; specifically, how easily does heat flow from the 2D materials where the heat is generated to various adjacent materials such as supporting substrates and metal contacts? This project will focus on the measurement of the thermal conductance across chalcogenide:metal and chalcogenide:complex oxide heterointerfaces via opto-thermal metrology techniques such as time-domain and frequency-domain thermoreflectance (TDTR, FDTR). This work will introduce the REU participants to the use of laser-based metrology for measuring electron and phonon transport properties on the nanoscale, building skills in a variety of areas beyond just nanoscale heat transfer including optics, electronics/signal processing and materials science.
Prof. Joan Redwing (MatSE/EE): Exploring 2D Ferroelectric Crystals and Thin Films
2D materials exhibit a range of intriguing optical and electronic properties and, recently, it was discovered that layered chalcogenides such as In2Se3also exhibit ferroelectricity in both in-plane and out-of-plane orientations. The development of van der Waals heterostructures based on 2D ferroelectrics opens up a wide range of possible new devices that take advantage of the tunability afforded by the ferroelectric layer. Initial studies of ferroelectricity were carried out using ultra-thin flakes of In2Se3exfoliated from bulk crystals but continued device development will require large area thin films. Recently we have succeeded in growing epitaxial thin films of In2Se3by metalorganic chemical vapor deposition (MOCVD) but the properties of the films in comparison to those of In2Se3 bulk single crystals are not well understood. The student on the project will characterize In2Se3 crystal flakes (grown by Bridgman melt-growth method) and thin films (grown by MOCVD) and compare their properties. It is hypothesized that grain boundaries present in In2Se3 thin films will not negatively impact ferroelectric properties out-of-plane. The student will test this hypothesis through a variety of characterization techniques including atomic force microscopy, Hall-effect measurements and polarization electric field (PE) measurements.
Prof. Thomas N. Jackson (EE): New Methods for Releasing and Combining Piezoelectric Films
Hypothesis: Released and stacked PZT thin films can provide superior performance compared to thicker films or bulk materials with the same total PZT thickness. Piezoelectric thin films based on materials like PbZr1-xTixO3 can only be deposited to a thickness of ~3 mm on Si substrates, due to stress-induced cracking. However, for applications such as energy harvesters and adjustable optics, thicker films are desirable. The REU student will work on methods for releasing thin films from the substrates on which they are grown, through the use of sacrificial layers such as ZnO and GeO2, so that they can be transferred to other substrates and stacked to prepare films with greater total PZT thickness. The student will characterize the properties and structural quality of the piezoelectric thick films, including the voltage drive requirements of stacked films compared to single layer devices.
Prof. Seong H. Kim (ChE): Surface Chemistry of Ferroelectric and 2D Materials
REU students will be involved in fundamental surface science studies of ferroelectric and 2D materials under various mechanical stresses. Ferroelectric materials under mechanical stress will develop surface charges that can alter the adsorption and reaction behaviors of molecules impinging from the gas phase. Similarly, local strains imposed on 2D materials due to substrate topography can alter the surface reactivity of 2D materials. Undergraduate students will work with graduate students to carry out spectroscopic analysis of gas adsorption isotherm and reactions at ferroelectric and 2D material surfaces. They will also use environment-controlled atomic force microscope and tribometer to induce various chemical reactions at these surfaces.
Prof. Susan B. Sinnott (MatSE): Investigate the Structure-Property Relationships for MXene Heterostructures
The proposed work is based on the hypothesis that heterostructures of MXenes will enable more rapid material design than production from a single MAX source. The properties of MXene heterostructures are currently not well established. Such heterostructures may be produced by layering different materials or by forming lateral junctions between similar structures with different compositions. A computational approach will be used at the level of density functional theory (DFT) using the projector augmented wave (PAW) in the Vienna Ab-initio Simulation Package (VASP) software. The choice of the exchange-correlational functional will be the Perdew-Burke-Ernzerhoff (PBE) generalized gradient functional. Hybrid functional calculations with HSE06 hybrid functionals will be used to obtain details of the electronic structure of the MXene heterostructures, as they provide a better determination of materials’ band gaps.
Prof. Bladimir Ramos-Alvarado (Mech E): Wetting Behavior and Interfacial Thermodynamics of 2D Materials
Classical molecular dynamics simulations and mean-field theory will be used to characterize the wettability of 2-D materials. The link between the thermodynamics of the interfacial liquid structure and observable macroscopic wetting behavior will be investigated. Graphene, hexagonal boron-nitride, and molybdenum disulfide will be the materials under consideration. The goal is to build the foundations for comprehensive models of solid-liquid interfaces to properly investigate interfacial transport processes at the nanoscale. The hypothesis behind this project is that surface chemistry and solid-liquid affinity, characterized via wetting behavior, hold the key to unravel the complexities of energy and momentum transport at interfaces. The undergraduate student participating in this project will be fully engaged in the modelling and analysis stages.
Nanofabrication and testing of materials and devices
Prof. Shengxi Huang (EE): 2D Material-Based Biosensing Platform
In this project, a biosensing platform will be built using 2D materials and optical spectroscopy. The hypotheses are: (1) 2D material substrate will enhance the Raman signals of biomolecules; (2) there is a selection rule between the biomolecule and 2D material to achieve the strongest enhancement. In this project, biomolecules will be deposited on the 2D material surface, followed by the optical spectroscopic measurement on the biomolecules, and 2D materials will induce signal enhancement for some molecules. We will explore the preferable combination of biomolecule and 2D material for biosensing application, and use modeling and calculations to explain the results. The REU student will learn the basics of 2D materials, biomolecules, and optical spectroscopy, get hands-on experience inpreparing few-layer 2D materials using mechanical exfoliation. The student will also perform optical spectroscopic measurement such as Raman spectroscopy, and analyze the signal using numerical fitting together with quantum theory.
Prof. Joshua A. Robinson (MATSE): Understanding the Electronic Properties of 2D Materials
The project will test the hypothesis that the choice of substrate heavily influences the electronic properties of 2D layers. To address this hypothesis, the student will focus on fabricating electronic test structures to extract the electrical properties of 2D materials grown on various different substrates and comparing the results to understand how substrate influences carrier mobility, channel doping, contact resistance, to name a few. Materials include the “famous” 2D material, graphene, as well as a variety of exotic 2D materials others may not know much about.
Prof. Saptarshi Das (ESM): Next Generation Ultra-Low Power and High-Performance Devices based on 2D Layered Materials
Discovery of graphene has led to the resurrection of an interesting class of materials – namely, 2D layered materials. Several such materials exist in nature and many more can be designed and engineered either through bottom up growth or top-down synthesis techniques. Among the 2D family, TMDCs have garnered significant interest in the device community owing to their exquisite electronic properties. For example, in monolayer form, the TMDCs are direct bandgap semiconductors fostering enhanced light-matter interaction and thereby enabling photonic and photovoltaic devices. TMDCs also exhibit spin hall effects, valley hall effects (VHE), quantum tunneling phenomena and existence of excitons and trions, which can lay the foundation for future ultra-low power spintronic, valleytronic and excitonic devices. Further, TMDCs can be flexed, stretched and even squeezed to enable flextronics, piezotronic and straintronic devices. Finally, the atomically thin body nature of these 2D materials allow superior electrostatic gate control, when used in an FET geometry, which make them promising candidates for high performance devices. In the Das Research Group, REU students will not only learn about innovative devices but also develop their skills in processing of various nanomaterials, different nanofabrication techniques and numerous device characterization tools.
Prof. Jun Zhu (Physics): Electronic Properties of Low-Dimensional Materials
REU students will be involved in the study of the electronic properties of low-dimensional materials and how the properties differ from bulk materials (graphene, hexagonal boron nitride, and layered transition metal dichalcogenides). REU students will be trained in the preparation of devices and learn how to characterize the few-layered material in collaboration with their mentor, using techniques found in the fabrication of nanostructured devices. They will be trained in lithography in the Nanofabrication facility at Penn State. The Zhu lab is interested in developing new lithographic techniques that push the boundary of the current state of the art. For example, how does the performance of a valleytronic device depend on its dimensions? The work of an REU student in the nanofab facility at Penn State forms an important component of that quest. Upon successful creation of a nano-device or a new recipe, students would then characterize its electrical properties using electrical transport measurements.