James R. Adleman

Heterogenous Catalysis Mediated by Plasmon Heating

We introduce a new method for performing and miniaturizing many types of heterogeneous catalysis involving nanoparticles. The method makes use of the plasmon resonance present in nanoscale metal catalysts to provide the necessary heat of reaction when illuminated with a low-power laser. We demonstrate our approach by reforming a flowing, liquid mixture of ethanol and water over gold nanoparticle catalysts in a microfluidic channel. Plasmon heating of the nanoparticles provides not only the heat of reaction but the means to generate both water and ethanol vapor locally over the catalysts, which in turn allows the chip and the fluid lines to remain at room temperature. The measured products of the reaction, CO(2), CO, and H(2), are consistent with catalytic steam reforming of ethanol. The approach, which we refer to as plasmon-assisted catalysis, is general and can be used with a variety of endothermic catalytic processes involving nanoparticles.

Trapping of dielectric particles with light-induced space-charge fields

Light-induced space-charge fields in lithium niobate crystals are used to trap and manipulate dielectric particles on the surface of such crystals. Without any external voltage source, strong field gradients are present in the proximity of the crystal surface. These are used to trap particles with diameters in the range between 100 nm and some tens of micrometers.

Chemical Separations by Bubble-Assisted Interphase Mass-Transfer

We show that when a small amount of heat is added close to a liquid-vapor interface of a captive gas bubble in a microchannel, interphase mass-transfer through the bubble can occur in a controlled manner with only a slight change in the temperature of the fluid. We demonstrate that this method, which we refer to as bubble-assisted interphase mass-transfer (BAIM), can be applied to interphase chemical separations, e.g., simple distillation, without the need for high temperatures, vacuum, or active cooling. Although any source of localized heating could be used, we illustrate BAIM with an all-optical technique that makes use of the plasmon resonance in an array of nanoscale metal structures that are incorporated into the channel to produce localized heating of the fluid when illuminated by a stationary low-power laser.

3-dimensional electrode patterning within a microfluidic channel using metal ion implantation

The application of electrical fields within a microfluidic channel enables many forms of manipulation necessary for lab-on-a-chip devices. Patterning electrodes inside the microfluidic channel generally requires multi-step optical lithography. Here, we utilize an ion-implantation process to pattern 3D electrodes within a fluidic channel made of polydimethylsiloxane (PDMS). Electrode structuring within the channel is achieved by ion implantation at a 40 degrees angle with a metal shadow mask. The advantages of three-dimensional structuring of electrodes within a fluidic channel over traditional planar electrode designs are discussed. Two possible applications are presented: asymmetric particles can be aligned in any of the three axial dimensions with electro-orientation; colloidal focusing and concentration within a fluidic channel can be achieved through dielectrophoresis. Demonstrations are shown with E. coli, a rod shaped bacteria, and indicate the potential that ion-implanted microfluidic channels have for manipulations in the context of lab-on-a-chip devices.

Holographic grating formation in a colloidal suspension of silver nanoparticles

Holographic gratings are recorded in colloidal suspensions of silver nanoparticles by utilizing interfering nanosecond pulses. The diffraction efficiency is measured with continuous-wave light. An instantaneous response together with a transient grating are observed: the nanoparticles absorb the pump light and heat up. Heat is transferred to the solvent, and a delayed thermal grating appears. The final decay time constant of this grating depends quadratically on the period length and has a typical value of 1 micros for grating spacings of several micrometers.

High-Current Gain Two-Dimensional MoS2-Base Hot-Electron Transistors

The vertical transport of nonequilibrium charge carriers through semiconductor heterostructures has led to milestones in electronics with the development of the hot-electron transistor. Recently, significant advances have been made with atomically sharp heterostructures implementing various two-dimensional materials. Although graphene-base hot-electron transistors show great promise for electronic switching at high frequencies, they are limited by their low current gain. Here we show that, by choosing MoS2 and HfO2 for the filter barrier interface and using a noncrystalline semiconductor such as ITO for the collector, we can achieve an unprecedentedly high-current gain (α ∼ 0.95) in our hot-electron transistors operating at room temperature. Furthermore, the current gain can be tuned over 2 orders of magnitude with the collector-base voltage albeit this feature currently presents a drawback in the transistor performance metrics such as poor output resistance and poor intrinsic voltage gain. We anticipate our transistors will pave the way toward the realization of novel flexible 2D material-based high-density, low-energy, and high-frequency hot-carrier electronic applications.

Spectrographic microfluidic memory

Recent advancements in micro- and nanoscale fluidic manipulation have enabled the development of a new class of tunable optical structures which are collectively referred to as optofluidic devices. In this paper we will introduce our recent work directed towards the development of a spectrographic optofluidic memory. Data encoding for the memory is based on creating spectrographic codes consisting of multiple species of photoluminescent nanoparticles at discrete intensity levels which are suspended in liquids. The data cocktails are mixed, delivered and stored using a series of soft and hard-lithography microfluidic structures. Semiconductor quantum dots are ideally suited for this application due to their narrow and size tunable emission spectra and consistent excitation wavelength. Both pressure driven and electrokinetic approaches to spectral code writing have been developed and will be experimentally demonstrated here. Novel techniques for data storage and readout are also discussed and demonstrated.

Dual-mode operation of 2D material-base hot electron transistors.

Vertical hot electron transistors incorporating atomically-thin 2D materials, such as graphene or MoS2, in the base region have been proposed and demonstrated in the development of electronic and optoelectronic applications. To the best of our knowledge, all previous 2D material-base hot electron transistors only considered applying a positive collector-base potential (VCB > 0) as is necessary for the typical unipolar hot-electron transistor behavior. Here we demonstrate a novel functionality, specifically a dual-mode operation, in our 2D material-base hot electron transistors (e.g. with either graphene or MoS2 in the base region) with the application of a negative collector-base potential (VCB < 0). That is, our 2D material-base hot electron transistors can operate in either a hot-electron or a reverse-current dominating mode depending upon the particular polarity of VCB. Furthermore, these devices operate at room temperature and their current gains can be dynamically tuned by varying VCB. We anticipate our multi-functional dual-mode transistors will pave the way towards the realization of novel flexible 2D material-based high-density and low-energy hot-carrier electronic applications.

Tuning parameters of metal ion implantation within a microfluidic channel

Applying electrical fields is a simple and versatile method to manipulate and reconfigure optofluidic devices. Several methods to apply electric fields using electrodes on polymers or in the context of lab-on-a-chip devices exist. In this paper, we utilize an ion-implanted process to pattern electrodes within a fluidic channel made of polydimethylsiloxane (PDMS). Electrode structuring within the channel is achieved by ion implantation at a 40° angle with a metal shadow mask. In previous work using the ion-implantation process, we demonstrated two possible applications in the context of lab-on-a-chip applications. Asymmetric particles were aligned through electro-orientation. Colloidal focusing and concentration was possible with negative dielectrophoresis. In this paper, we discuss the different electrode structures that are possible by changing the channel dimensions. A second parameter of ion implantation dosage prevents the shorting of electrodes on the side wall or top wall of the fluidic channel to the bottom. This allows for floating electrodes on the side wall or top wall. These type of electrodes help prevent electrolysis as the liquid is not in direct contact with the voltage source. Possible applications of the different electrode structures that are possible are discussed.

Holographic grating formation in silver nanoparticle suspensions

Thermal gratings are recorded in silver nanoparticle suspensions by nanosecond pulsed holography. Initial transients in diffraction efficiency demonstrate competing effects in grating formation. The gratingpsilas final decay is consistent with the suspensionpsilas thermal conductivity.