Zinc telluride (ZnTe) is an attractive II-VI compound semiconductor with a direct

bandgap of 2.26 eV that is used in many applications in optoelectronic devices. Compared

to the two dimensional (2D) thin-film semiconductors, one-dimensional (1D)

nanowires can have different electronic properties for potential novel applications.

In this work, we present the study of ZnTe nanowires (NWs) that are synthesized

through a simple vapor-liquid-solid (VLS) method. By controlling the presence or

the absence of Au catalysts and controlling the growth parameters such as growth

temperature, various growth morphologies of ZnTe, such as thin films and nanowires

can be obtained. The characterization of the ZnTe nanostructures and films was

performed using scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy

(EDX), high- resolution transmission electron microscope (HRTEM), X-ray

diffraction (XRD), photoluminescence (PL), Raman spectroscopy and light scattering

measurement. After confirming the crystal purity of ZnTe, two-terminal diodes and

three-terminal transistors were fabricated with both nanowire and planar nano-sheet

configurations, in order to correlate the nanostructure geometry to device performance

including field effect mobility, Schottky barrier characteristics, and turn-on

characteristics. Additionally, optoelectronic properties such as photoconductive gain

and responsivity were compared against morphology. Finally, ZnTe was explored in

conjunction with ZnO in order to form type-II band alignment in a core-shell nanostructure.

Various characterization techniques including scanning electron microscopy,

energy-dispersive X-ray spectroscopy , x-ray diffraction, Raman spectroscopy, UV-vis

reflectance spectra and photoluminescence were used to investigate the modification

of ZnO/ZnTe core/shell structure properties. In PL spectra, the eliminated PL intensity

of ZnO wires is primarily attributed to the efficient charge transfer process

occurring between ZnO and ZnTe, due to the band alignment in the core/shell structure. Moreover, the result of UV-vis reflectance spectra corresponds to the band

gap energy of ZnO and ZnTe, respectively, which confirm that the sample consists of

ZnO/ZnTe core/shell structure of good quality.

In the past decades, single-cell metabolic analysis has been playing a key role in understanding cellular heterogeneity, disease initiation, progression, and drug resistance. Therefore, it is critical to develop technologies for individual cellular metabolic analysis using various configurations of microfluidic devices. Compared to bulk-cell analysis which is widely used by reporting an averaged measurement, single-cell analysis is able to present the individual cellular responses to the external stimuli. Particularly, oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) are two key parameters to monitor heterogeneous metabolic profiles of cancer cells. To achieve multi-parameter metabolic measurements on single cells, several technical challenges need to be overcome: (1) low adhesion of soft materials micro-fabricated on glass surface for multiple-sensor deposition and single-cell immobilization, e.g. SU-8, KMPR, etc.; (2) high risk of using external mechanical forces to create hermetic seals between two rigid fused silica parts, even with compliance layers; (3) how to accomplish high-throughput for single-cell trapping, metabolic profiling and drug screening; (4) high process cost of micromachining on glass substrate and incapability of mass production.

In this dissertation, the development of microfabrication technologies is demonstrated to design reliable configurations for analyzing multiple metabolic parameters from single cells, including (1) improved KMPR/SU-8 microfabrication protocols for fabricating microwell arrays that can be integrated and sealed to 3 × 3 tri-color sensor arrays for OCR and ECAR measurements; (2) design and characterization of a microfluidic device enabling rapid single-cell trapping and hermetic sealing single cells and tri-color sensors within 10 × 10 hermetically sealed microchamber arrays; (3) exhibition of a low-cost microfluidic device based on plastics for single-cell metabolic multi-parameter profiling. Implementation of these improved microfabrication methods should address the aforementioned challenges and provide a high throughput and multi-parameter single cell metabolic analysis platform.

A Microbial fuel cell (MFC) is a bio-inspired carbon-neutral, renewable electrochemical converter to extract electricity from catabolic reaction of micro-organisms. It is a promising technology capable of directly converting the abundant biomass on the planet into electricity and potentially alleviate the emerging global warming and energy crisis. The current and power density of MFCs are low compared with conventional energy conversion techniques. Since its debut in 2002, many studies have been performed by adopting a variety of new configurations and structures to improve the power density. The reported maximum areal and volumetric power densities range from 19 mW/m2 to 1.57 W/m2 and from 6.3 W/m3 to 392 W/m3, respectively, which are still low compared with conventional energy conversion techniques. In this dissertation, the impact of scaling effect on the performance of MFCs are investigated, and it is found that by scaling down the characteristic length of MFCs, the surface area to volume ratio increases and the current and power density improves. As a result, a miniaturized MFC fabricated by Micro-Electro-Mechanical System(MEMS) technology with gold anode is presented in this dissertation, which demonstrate a high power density of 3300 W/m3. The performance of the MEMS MFC is further improved by adopting anodes with higher surface area to volume ratio, such as carbon nanotube (CNT) and graphene based anodes, and the maximum power density is further improved to a record high power density of 11220 W/m3. A novel supercapacitor by regulating the respiration of the bacteria is also presented, and a high power density of 531.2 A/m2 (1,060,000 A/m3) and 197.5 W/m2 (395,000 W/m3), respectively, are marked, which are one to two orders of magnitude higher than any previously reported microbial electrochemical techniques.

A single cell is the very fundamental element in an organism; however, it contains the most complicated and stochastic information, such as DNA, RNA, and protein expression. Thus, it is a necessity to study stochastic gene expression in order to discover the biosignatures at the single-cell level. The heterogeneous gene expression of single cells from an isogenic cell population has already been studied for years. Yet to date, single-cell studies have been confined in a fashion of analyzing isolated single cells or a dilution of cells from the bulk-cell populations. These techniques or devices are limited by either the mechanism of cell lysis or the difficulties to target specific cells without harming neighboring cells.

This dissertation presents the development of a laser lysis chip combined with a two-photon laser system to perform single-cell lysis of single cells in situ from three-dimensional (3D) cell spheroids followed by analysis of the cell lysate with two-step reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The 3D spheroids were trapped in a well in the custom-designed laser lysis chip. Next, each single cell of interest in the 3D spheroid was identified and lysed one at a time utilizing a two-photon excited laser. After each cell lysis, the contents inside the target cell were released to the surrounding media and carried out to the lysate collector. Finally, the gene expression of each individual cell was measured by two-step RT-qPCR then spatially mapped back to its original location in the spheroids to construct a 3D gene expression map.

This novel technology and approach enables multiple gene expression measurements in single cells of multicellular organisms as well as cell-to-cell heterogeneous responses to the environment with spatial recognition. Furthermore, this method can be applied to study precancerous tissues for a better understanding of cancer progression and for identifying early tumor development.

This work demonstrates novel nBn photodetectors including mid-wave infrared (MWIR) nBn photodetectors based on InAs/InAsSb type-II superlattices (T2SLs) with charge as the output signal, and visible nBn photodetectors based on CdTe with current output. Furthermore, visible/MWIR two-color photodetectors (2CPDs) are fabricated through monolithic integration of the CdTe nBn photodetector and an InSb photodiode.

The MWIR nBn photodetectors have a potential well for holes present in the barrier layer. At low voltages of < −0.2 V, which ensure low dark current <10-5 A/cm2 at 77 K, photogenerated holes are collected in this well with a storage lifetime of 40 s. This charge collection process is an in-device signal integration process that reduces the random noise significantly. Since the stored holes can be readout laterally as in charge-coupled devices, it is therefore possible to make charge-output nBn with much lower noise than conventional current-output nBn photodetectors.

The visible nBn photodetectors have a CdTe absorber layer and a ZnTe barrier layer with an aligned valence band edge. By using a novel ITO/undoped-CdTe top contact design, it has achieved a high specific detectivity of 3×1013 cm-Hz1/2/W at room temperature. Particularly, this CdTe nBn photodetector grown on InSb substrates enables the monolithic integration of CdTe and InSb photodetectors, and provides a platform to study in-depth device physics of nBn photodetectors at room temperature.

Furthermore, the visible/MWIR 2CPD has been developed by the monolithic integration of the CdTe nBn and an InSb photodiode through an n-CdTe/p-InSb tunnel junction. At 77 K, the photoresponse of the 2CPD can be switched between a 1-5.5 μm MWIR band and a 350-780 nm visible band by illuminating the device with an external light source or not, and applying with proper voltages. Under optimum conditions, the 2CPD has achieved a MWIR peak responsivity of 0.75 A/W with a band rejection ratio (BRR) of 52 dB, and a visible peak responsivity of 0.3 A/W with a BRR of 18 dB. This 2CPD has enabled future compact image sensors with high fill-factor and responsivity switchable between visible and MWIR colors.

Sb-based type-II superlattices (T2SLs) are potential alternative to HgCdTe for infrared detection due to their low manufacturing cost, good uniformity, high structural stability, and suppressed Auger recombination. The emerging InAs/InAsSb T2SLs have minority carrier lifetimes 1-2 orders of magnitude longer than those of the well-studied InAs/InGaSb T2SLs, and therefore have the potential to achieve photodetectors with higher performance. This work develops a novel method to measure the minority carrier lifetimes in infrared materials, and reports a comprehensive characterization of minority carrier lifetime and transport in InAs/InAsSb T2SLs at temperatures below 77 K.

A real-time baseline correction (RBC) method for minority carrier lifetime measurement is developed by upgrading a conventional boxcar-based time-resolved photoluminescence (TRPL) experimental system that suffers from low signal-to-noise ratio due to strong low frequency noise. The key is to modify the impulse response of the conventional TRPL system, and therefore the system becomes less sensitive to the dominant noise. Using this RBC method, the signal-to-noise ratio is improved by 2 orders of magnitude.

A record long minority carrier lifetime of 12.8 μs is observed in a high-quality mid-wavelength infrared InAs/InAsSb T2SLs at 15 K. It is further discovered that this long lifetime is partially due to strong carrier localization, which is revealed by temperature-dependent photoluminescence (PL) and TRPL measurements for InAs/InAsSb T2SLs with different period thicknesses. Moreover, the PL and TRPL results suggest that the atomic layer thickness variation is the main origin of carrier localization, which is further confirmed by a calculation using transfer matrix method.

To study the impact of the carrier localization on the device performance of InAs/InAsSb photodetectors, minority hole diffusion lengths are determined by the simulation of external quantum efficiency (EQE). A comparative study shows that carrier localization has negligible effect on the minority hole diffusion length in InAs/InAsSb T2SLs, and the long minority carrier lifetimes enhanced by carrier localization is not beneficial for photodetector operation.

InAsBi is a narrow direct gap III-V semiconductor that has recently attracted considerable attention because its bandgap is tunable over a wide range of mid- and long-wave infrared wavelengths for optoelectronic applications. Furthermore, InAsBi can be integrated with other III-V materials and is potentially an alternative to commercial II-VI photodetector materials such as HgCdTe.

Several 1 μm thick, nearly lattice-matched InAsBi layers grown on GaSb are examined using Rutherford backscattering spectrometry and X-ray diffraction. Random Rutherford backscattering measurements indicate that the average Bi mole fraction ranges from 0.0503 to 0.0645 for the sample set, and ion channeling measurements indicate that the Bi atoms are substitutional. The X-ray diffraction measurements show a diffraction sideband near the main (004) diffraction peak, indicating that the Bi mole fraction is not laterally uniform in the layer. The average out of plane tetragonal distortion is determined by modeling the main and sideband diffraction peaks, from which the average unstrained lattice constant of each sample is determined. By comparing the Bi mole fraction measured by random Rutherford backscattering with the InAsBi lattice constant for the sample set, the lattice constant of zinc blende InBi is determined to be 6.6107 Å.

Several InAsBi quantum wells tensilely strained to the GaSb lattice constant with dilute quantities of Bi are characterized using photoluminescence spectroscopy. Investigation of the integrated intensity as a function of carrier excitation density spanning 5×1025 to 5×1026 cm-3 s-1 indicates radiative dominated recombination and high quantum efficiency over the 12 to 250 K temperature range. The bandgap of InAsBi is ascertained from the photoluminescence spectra and parameterized as a function of temperature using the Einstein single oscillator model. The dilute Bi mole fraction of the InAsBi quantum wells is determined by comparing the measured bandgap energy to that predicted by the valence band anticrossing model. The Bi mole fraction determined by photoluminescence agrees reasonably well with that estimated using secondary ion mass spectrometry.

Mechanical properties of cells are important in maintaining physiological functions of biological systems. Quantitative measurement and analysis of mechanical properties can help understand cellular mechanics and its functional relevance and discover physical biomarkers for diseases monitoring and therapeutics.

This dissertation presents a work to develop optical methods for studying cell mechanics which encompasses four applications. Surface plasmon resonance microscopy based optical method has been applied to image intracellular motions and cell mechanical motion. This label-free technique enables ultrafast imaging with extremely high sensitivity in detecting cell deformation. The technique was first applied to study intracellular transportation. Organelle transportation process and displacement steps of motor protein can be tracked using this method. The second application is to study heterogeneous subcellular membrane displacement induced by membrane potential (de)polarization. The application can map the amplitude and direction of cell deformation. The electromechanical coupling of mammalian cells was also observed. The third application is for imaging electrical activity in single cells with sub-millisecond resolution. This technique can fast record actions potentials and also resolve the fast initiation and propagation of electromechanical signals within single neurons. Bright-field optical imaging approach has been applied to the mechanical wave visualization that associated with action potential in the fourth application. Neuron-to-neuron viability of membrane displacement was revealed and heterogeneous subcellular response was observed.

All these works shed light on the possibility of using optical approaches to study millisecond-scale and sub-nanometer-scale mechanical motions. These studies revealed ultrafast and ultra-small mechanical motions at the cellular level, including motor protein-driven motions and electromechanical coupled motions. The observations will help understand cell mechanics and its biological functions. These optical approaches will also become powerful tools for elucidating the interplay between biological and physical functions.

To date, the most popular and dominant material for commercial solar cells is

crystalline silicon (or wafer-Si). It has the highest cell efficiency and cell lifetime out

of all commercial solar cells. Although the potential of crystalline-Si solar cells in

supplying energy demands is enormous, their future growth will likely be constrained

by two major bottlenecks. The first is the high electricity input to produce

crystalline-Si solar cells and modules, and the second is the limited supply of silver

(Ag) reserves. These bottlenecks prevent crystalline-Si solar cells from reaching

terawatt-scale deployment, which means the electricity produced by crystalline-Si

solar cells would never fulfill a noticeable portion of our energy demands in the future.

In order to solve the issue of Ag limitation for the front metal grid, aluminum (Al)

electroplating has been developed as an alternative metallization technique in the

fabrication of crystalline-Si solar cells. The plating is carried out in a

near-room-temperature ionic liquid by means of galvanostatic electrolysis. It has been

found that dense, adherent Al deposits with resistivity in the high 10^–6 ohm-cm range

can be reproducibly obtained directly on Si substrates and nickel seed layers. An

all-Al Si solar cell, with an electroplated Al front electrode and a screen-printed Al

back electrode, has been successfully demonstrated based on commercial p-type

monocrystalline-Si solar cells, and its efficiency is approaching 15%. Further

optimization of the cell fabrication process, in particular a suitable patterning

technique for the front silicon nitride layer, is expected to increase the efficiency of

the cell to ~18%. This shows the potential of Al electroplating in cell metallization is

promising and replacing Ag with Al as the front finger electrode is feasible.

Programmable metallization cell (PMC) technology employs the mechanisms of metal ion transport in solid electrolytes (SE) and electrochemical redox reactions in order to form metallic electrodeposits. When a positive bias is applied to an anode opposite to a cathode, atoms at the anode are oxidized to ions and dissolve into the SE. Under the influence of the electric field, the ions move to the cathode and become reduced to form the electrodeposits. These electrodeposits are filamentary in nature and persistent, and since they are metallic can alter the physical characteristics of the material on which they are formed. PMCs can be used as next generation memories, radio frequency (RF) switches and physical unclonable functions (PUFs).

The morphology of the filaments is impacted by the biasing conditions. Under a relatively high applied electric field, they form as dendritic elements with a low fractal dimension (FD), whereas a low electric field leads to high FD features. Ion depletion effects in the SE due to low ion diffusivity/mobility also influences the morphology by limiting the ion supply into the growing electrodeposit.

Ion transport in SE is due to hopping transitions driven by drift and diffusion force. A physical model of ion hopping with Brownian motion has been proposed, in which the ion transitions are random when time window is larger than characteristic time. The random growth process of filaments in PMC adds entropy to the electrodeposition, which leads to random features in the dendritic patterns. Such patterns has extremely high information capacity due to the fractal nature of the electrodeposits.

In this project, lateral-growth PMCs were fabricated, whose LRS resistance is less than 10Ω, which can be used as RF switches. Also, an array of radial-growth PMCs was fabricated, on which multiple dendrites, all with different shapes, could be grown simultaneously. Those patterns can be used as secure keys in PUFs and authentication can be performed by optical scanning.

A kinetic Monte Carlo (KMC) model is developed to simulate the ion transportation in SE under electric field. The simulation results matched experimental data well that validated the ion hopping model.