Urbana, IL, United States
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Choi W.,Micro and Nanotechnology Laboratory | Seabron E.,University of Illinois at Urbana - Champaign | Mohseni P.K.,Micro and Nanotechnology Laboratory | Kim J.D.,University of Illinois at Urbana - Champaign | And 9 more authors.
ACS Nano | Year: 2017

Selective lateral epitaxial (SLE) semiconductor nanowires (NWs), with their perfect in-plane epitaxial alignment, ability to form lateral complex p-n junctions in situ, and compatibility with planar processing, are a distinctive platform for next-generation device development. However, the incorporation and distribution of impurity dopants in these planar NWs via the vapor-liquid-solid growth mechanism remain relatively unexplored. Here, we present a detailed study of SLE planar GaAs NWs containing multiple alternating axial segments doped with Si and Zn impurities by metalorganic chemical vapor deposition. The dopant profile of the lateral multi-p-n junction GaAs NWs was imaged simultaneously with nanowire topography using scanning microwave impedance microscopy and correlated with infrared scattering-type near-field optical microscopy. Our results provide unambiguous evidence that Zn dopants in the periodically twinned and topologically corrugated p-type segments are preferentially segregated at twin plane boundaries, while Si impurity atoms are uniformly distributed within the n-type segments of the NWs. These results are further supported by microwave impedance modulation microscopy. The density functional theory based modeling shows that the presence of Zn dopant atoms reduces the formation energy of these twin planes, and the effect becomes significantly stronger with a slight increase of Zn concentration. This implies that the twin formation is expected to appear when a threshold planar concentration of Zn is achieved, making the onset and twin periodicity dependent on both Zn concentration and nanowire diameter, in perfect agreement with our experimental observations. © 2017 American Chemical Society.


Park K.,Micro and Nanotechnology Laboratory | Millet L.J.,Micro and Nanotechnology Laboratory | Popescu G.,Micro and Nanotechnology Laboratory | Popescu G.,University of Illinois at Urbana - Champaign | And 4 more authors.
Proceedings of the National Academy of Sciences of the United States of America | Year: 2010

The characterization of physical properties of cells such as their mass and stiffness has been of great interest and can have profound implications in cell biology, tissue engineering, cancer, and disease research. For example, the direct dependence of cell growth rate on cell mass for individual adherent human cells can elucidate the mechanisms underlying cell cycle progression. Here we develop an array of micro-electro-mechanical systems (MEMS) resonant mass sensors that can be used to directly measure the biophysical properties, mass, and growth rate of single adherent cells. Unlike conventional cantilever mass sensors, our sensors retain a uniform mass sensitivity over the cell attachment surface. By measuring the frequency shift of the mass sensors with growing (soft) cells and fixed (stiff) cells, and through analytical modeling, we derive the Young's modulus of the unfixed cell and unravel the dependence of the cell mass measurement on cell stiffness. Finally, we grew individual cells on the mass sensors and measured their mass for 50+ hours. Our results demonstrate that adherent human colon epithelial cells have increased growth rates with a larger cell mass, and the average growth rate increases linearly with the cell mass, at 3.25%/hr. Our sensitive mass sensors with a position-independent mass sensitivity can be coupled with microscopy for simultaneous monitoring of cell growth and status, and provide an ideal method to study cell growth, cell cycle progression, differentiation, and apoptosis.


Zhou J.,University of Illinois at Urbana - Champaign | Wang F.,University of Illinois at Urbana - Champaign | Saif T.A.,Micro and Nanotechnology Laboratory
Biophysical Journal | Year: 2010

Cancer deaths are primarily caused by metastases, not by the parent tumor. During metastasis, malignant cells detach from the parent tumor, and spread through the circulatory system to invade new tissues and organs. The physical-chemical mechanisms and parameters within the cellular microenvironment that initiate the onset of metastasis, however, are not understood. Here we show that human colon carcinoma (HCT-8) cells can exhibit a dissociative, metastasis-like phenotype (MLP) in vitro when cultured on substrates with appropriate mechanical stiffness. This rather remarkable phenotype is observed when HCT-8 cells are cultured on gels with intermediate-stiffness (physiologically relevant 21-47 kPa), but not on very soft (1 kPa) and very stiff (3.6 GPa) substrates. The cell-cell adhesion molecule E-Cadherin, a metastasis hallmark, decreases 4.73 ± 1.43 times on cell membranes in concert with disassociation. Both specific and nonspecific cell adhesion decrease once the cells have disassociated. After reculturing the disassociated cells on fresh substrates, they retain the disassociated phenotype regardless of substrate stiffness. Inducing E-Cadherin overexpression in MLP cells only partially reverses theMLP phenotype in a minority population of the dissociated cells. This important experiment reveals that E-Cadherin does notplay a significant role in the upstream regulation of the mechanosensing cascade. Our results indicate, during culture on theappropriate mechanical microenvironment, HCT-8 cells undergo a stable cell-state transition with increased in vitro metastasis- like characteristics as compared to parent cells grown on standard, very stiff tissue culture dishes. Nuclear staining reveals that a large nuclear deformation (major/minor axis ratio, 2:5) occurs in HCT-8 cells when cells are cultured on polystyrene substrates, but it is markedly reduced (ratio, 1:3) in cells grown on 21 kPa substrates, suggesting the cells are experiencing different intracellular forces when grown on stiff as compared to soft substrates. Furthermore, MLP can be inhibited by blebbistatin,which inactivates myosin II activity and relaxes intracellular forces. This novel finding suggests that the onset of metastasis may, in part, be linked to the intracellular forces and the mechanical microenvironment of the tumor. © 2010 by the Biophysical Society.


Mathias P.C.,Micro and Nanotechnology Laboratory | Jones S.I.,102 S. Goodwin Ave. | Wu H.-Y.,Micro and Nanotechnology Laboratory | Yang F.,Micro and Nanotechnology Laboratory | And 5 more authors.
Analytical Chemistry | Year: 2010

DNA microarrays are used to profile changes in gene expression between samples in a high-throughput manner, but measurements of genes with low expression levels can be problematic with standard microarray substrates. In this work, we expand the detection capabilities of a standard microarray experiment using a photonic crystal (PC) surface that enhances fluorescence observed from microarray spots. This PC is inexpensively and uniformly fabricated using a nanoreplica molding technique, with very little variation in its optical properties within- and between-devices. By using standard protocols to process glass microarray substrates in parallel with PCs, we evaluated the impact of this substrate on a one-color microarray experiment comparing gene expression in two developmental stages of Glycine max. The PCs enhanced the signal-to-noise ratio observed from microarray spots by 1 order of magnitude, significantly increasing the number of genes detected above substrate fluorescence noise. PC substrates more than double the number of genes classified as differentially expressed, detecting changes in expression even for low expression genes. This approach increases the dynamic range of a surface-bound fluorescence-based assay to reliably quantify small quantities of DNA that would be impossible with standard substrates. © 2010 American Chemical Society.


Neff A.L.,University of Illinois at Urbana - Champaign | Allain J.P.,University of Illinois at Urbana - Champaign | Allain J.P.,Micro and Nanotechnology Laboratory | Bedoya F.,University of Illinois at Urbana - Champaign | And 2 more authors.
Journal of Nuclear Materials | Year: 2015

Abstract Lithium wall conditioning on PFCs (Plasma Facing Components) on a variety of substrate platforms (e.g. graphite, Mo, etc.) has resulted in improved plasma performance on multiple magnetic fusion devices. On graphite, this improvement occurs through the control of retention and recycling of hydrogen at the plasma-material interface by the chemical bonding of Li, O, and D at the surface. Moderate fluence (1 × 1021 m-2) studies of Li on W, performed in PRIHSM (Particle Radiation in Soft and Hard Matter), demonstrated that H retention is similar to Li on ATJ graphite but He ions, when mixed in a D beam, can inhibit the retention. To expand these studies closer to reactor relevant regimes like inside ITER, irradiations were carried out in Magnum-PSI at DIFFER up to fluences of ∼1025 m-2 with D, He, and He-seeded D plasmas (He 5-10%). Results show that D is still retained at higher fluxes and fluences. © 2014 Elsevier B.V.


Reddy R.,University of Illinois at Urbana - Champaign | Mayerich D.,University of Illinois at Urbana - Champaign | Walsh M.,University of Illinois at Urbana - Champaign | Schulmerich M.,University of Illinois at Urbana - Champaign | And 3 more authors.
Proceedings - International Symposium on Biomedical Imaging | Year: 2012

Fourier transform infrared (FT-IR) spectroscopic imaging provides spatially resolved chemical information. Recent developments have shown that this chemical information can be used to determine tissue cell types. Our goal is then to use the spatial distribution of tissue cell types to perform accurate diagnosis of cancer. However, this step is limited by the spatial resolution provided by current imaging systems. In this paper, we demonstrate that these instruments can be designed to provide better spatial resolution for tissue chemistry. We present an optical model for the propagation of light through an FT-IR spectroscopic imaging system. Using this model, we minimally modify an existing FT-IR spectroscopic imaging system to obtain significantly higher resolution and image quality. We demonstrate that it is possible to identify previously obscured tissue types by performing histological classification based on bio-chemically derived spectral features (metrics). © 2012 IEEE.


Banerjee S.,Micro and Nanotechnology Laboratory | Banerjee S.,University of Illinois at Urbana - Champaign | Wilson J.,University of Illinois at Urbana - Champaign | Shim J.,Micro and Nanotechnology Laboratory | And 7 more authors.
Advanced Functional Materials | Year: 2015

Slowing down DNA translocation speed in a nanopore is essential to ensuring reliable resolution of individual bases. Thin membrane materials enhance spatial resolution but simultaneously reduce the temporal resolution as the molecules translocate far too quickly. In this study, the effect of exposed graphene layers on the transport dynamics of both single (ssDNA) and double-stranded DNA (dsDNA) through nanopores is examined. Nanopore devices with various combinations of graphene and Al2O3 dielectric layers in stacked membrane structures are fabricated. Slow translocations of ssDNA in nanopores drilled in membranes with layers of graphene are reported. The increased hydrophobic interactions between the ssDNA and the graphene layers could explain this phenomenon. Further confirmation of the hydrophobic origins of these interactions is obtained through reporting significantly faster translocations of dsDNA through these graphene layered membranes. Molecular dynamics simulations confirm the preferential interactions of DNA with the graphene layers as compared to the dielectric layer verifying the experimental findings. Based on our findings, we propose that the integration of multiple stacked graphene layers could slow down DNA enough to enable the identification of nucleobases. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.


Xiong F.,University of Illinois at Urbana - Champaign | Xiong F.,Micro and Nanotechnology Laboratory | Liao A.,University of Illinois at Urbana - Champaign | Liao A.,Micro and Nanotechnology Laboratory | And 4 more authors.
Device Research Conference - Conference Digest, DRC | Year: 2010

Phase-change materials (PCM) like Ge2Sb2Te 5 (GST) have been proposed for non-volatile memory and reconfigurable electronics [1, 2]. One of the drawbacks associated with this technology is the relatively high (∼0.5 mA) programming currents required [3]. In this work, we utilize carbon nanotubes (CNTs) as nano-scaled interconnects to induce reversible switching in very small GST bits (∼10 nm). This lowers the programming currents to <10 μA, two orders of magnitude less than state-of-the-art PCM devices. Our work provides a novel method to build very low power non-volatile memory. It also shows that GST can be used to "repair" broken CNT connections, paving the way toward reconfigurable CNT electronics. © 2010 IEEE.


Salm E.,University of Illinois at Urbana - Champaign | Salm E.,Micro and Nanotechnology Laboratory | Zhong Y.,Micro and Nanotechnology Laboratory | Zhong Y.,University of Illinois at Urbana - Champaign | And 9 more authors.
Analytical Chemistry | Year: 2014

Electrical detection of nucleic acid amplification through pH changes associated with nucleotide addition enables miniaturization, greater portability of testing apparatus, and reduced costs. However, current ion-sensitive field effect transistor methods for sensing nucleic acid amplification rely on establishing the fluid gate potential with a bulky, difficult to microfabricate reference electrode that limits the potential for massively parallel reaction detection. Here we demonstrate a novel method of utilizing a microfabricated solid-state quasi-reference electrode (QRE) paired with a pH-insensitive reference field effect transistor (REFET) for detection of real-time pH changes. The end result is a 0.18 μm, silicon-on-insulator, foundry-fabricated sensor that utilizes a platinum QRE to establish a pH-sensitive fluid gate potential and a PVC membrane REFET to enable pH detection of loop mediated isothermal amplification (LAMP). This technique is highly amendable to commercial scale-up, reduces the packaging and fabrication requirements for ISFET pH detection, and enables massively parallel droplet interrogation for applications, such as monitoring reaction progression in digital PCR. © 2014 American Chemical Society.

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