Kang-Hoon Choi

Simultaneous atomic force microscope and quartz crystal microbalance measurements: Investigation of human plasma fibrinogen adsorption

We have combined the tapping-mode atomic force microscope (AFM) and quartz crystal microbalance (QCM) for simultaneous investigation of human plasma fibrinogen adsorption on a metallic surface using these two instruments. The AFM images show the surface changes with molecular resolution while the corresponding resonance frequency shift of the QCM provides quantitative adsorbed mass estimates over the whole sensing area. The combination of AFM with QCM allowing the simultaneous measurements with two techniques working at very different scales and probing different properties of the adsorbed layer provides quantitative and qualitative information that can distinguish different protein adsorption mechanisms.

Combined atomic force microscope and acoustic wave devices: Application to electrodeposition

We present a combination of acoustic wave based sensors with scanning probe microscopy as a tool for better understanding the interaction of the former with the surrounding viscous medium when used for detection of analytes in liquids. Simultaneous analysis of the gold coated sensing surface with an atomic force microscope and monitoring changes in the acoustic propagation properties during copper electrodeposition provides a mean of correlating observations on the nanometer and millimeter scales. We find that the frequency shift of the quartz crystal microbalance is predominantly attributed to viscous effects in the lower mass range (below 1 μg/cm2 copper electrodeposition) and only becomes representative of the added rigid mass in the higher mass range. We observe that the sensitivity of surface acoustic wave Love-mode devices appears constant over the whole mass range analyzed (0.5–10 μg/cm2), indicating a rigid layer interaction leading to a frequency shift representative of the deposited mass.

Simultaneous AFM and QCM Measurements Methodology Validation Using Electrodeposition

We assess the validity and advantages of using a quartz crystal microbalance ~QCM! as the metallic-coated substrate used for atomic force microscopy ~AFM! measurements by studying two well-known electrochemical reactions, silver electrodeposition on gold and copper electrodeposition on gold. We compare the results provided by electrochemistry ~cyclic voltammetry!, QCM frequency, and damping variations as well as AFM topography, and analyze the advantages of combining the three methods in the same instrument. Comparison of the evolution of the frequency of the third and fifth QCM overtones allows identification of the type of interaction between the sensing electrode and its environment: a rigid layer when the frequency shift is proportional to the overtone number, viscous interaction when the frequency shift is proportional to the square root of the overtone number. This identification scheme leads to results confirmed by the QCM damping.

Simultaneous atomic force microscope and quartz crystal microbalance measurements - Interactions and displacement field of a quartz crystal microbalance

We analyze the interaction of two instruments often used in material science analysis, the atomic force microscope (AFM) and the quartz crystal microbalance (QCM), here combined in a single instrument for simultaneous measurements on a single sample. We show, using finite element analysis, that the in-plane displacement of a QCM oscillating in liquid with a quality factor of 2000 is 2 nm. The out-of-plane displacement is about one tenth of the in-plane displacement. This latter effect, due to the finite size of the electrodes, results in longitudinal acoustic waves launched in the liquid surrounding the QCM. If bounced against an obstacle, in our case the AFM cantilever holder, these longitudinal waves create standing wave patterns which cause frequency fluctuations of the resonator when it is moved, and thus decrease the QCM sensitivity.

Investigation of protein adsorption with simultaneous measurements of atomic force microscope and quartz crystal microbalance

We have combined the tapping-mode atomic force microscope (AFM) and quartz crystal microbalance (QCM) for simultaneous investigation and characterization of protein adsorption on various metallic surfaces using these two instruments. The adsorption of proteins such as human plasma fibrinogen and anti-human immunoglobulin onto the metal or oxide/QCM surface were monitored using both methods at the same time when varying the concentration of proteins. The combination of AFM with QCM allowing the simultaneous measurements with two techniques working at very different scales and probing different properties of the adsorbed layer provides quantitative and qualitative information that can distinguish different protein adsorption behavior.

High electron mobility in AlGaN/GaN HEMT grown on sapphire: strain modification by means of AlN interlayers.

The performance of AlGaN/GaN High Electron Mobility (HEMT) transistors is directly related to the electrical characteristics of the two-dimensional electron gas formed at the interface thanks to the piezoelectric field. Modification of the Al content or thickness of the AlGaN layer can within a certain limit modify the carrier density and mobility in the 2DEG. However, further reduction of the sheet resistance requires strain engineering of the heterostructure. An effective way to reduce the sheet resistance, as well as to lower the threading dislocation (TD) density, is to perform strain engineering through the use of low temperature AlN interlayers inserted in the GaN buffer layer. From correlation of AFM, TEM and HRXRD mapping of the HEMT layers, the strain modification, as well as the mechanism reducing the TD density, can be explained by the highly defected nature of the AlN interlayer grown at low temperature, as well as its very small thickness. The LT AlN acts as a second nucleation layer for the GaN grown on top. Contrarily, when the AlN interlayer is grown at 1050°C, its high crystalline quality and the possibility to grow pseudomorphic and abrupt interfaces, allows its use at the AlGaN/GaN interface. Optimal combination of the AlGaN and AlN layer thickness leads to record values of the mobility at room temperature of 2050 cm2/Vs, for heterostructures grown on sapphire, which is approaching state-of-the-art for HEMT grown on SiC.

Release of epitaxial layers grown on InAs substrates

We present a process for selectively releasing large area epitaxial layers from InAs substrates. The technique employs the extreme selectivity between InAs and GaSb when etched by hydrochloric acid. The etch-stop layer isa strain-compensated superlattice containing GaSb. It is sandwiched between the desired epitaxial layer and the InAs substrate to effectively stop the etch process, leaving the desired epitaxial layer with high surface and crystal quality after the process. The etch-stop layer can be further etched away. The epitaxial thin film had been successfully grafted to a GaAs substrate by van der Waals bonding.