Research Projects

Current Projects: Porous Silicon Nanoparticle Therapeutics | Porous Silicon Bloch Surface and Sub-surface Waves Biosensors | Photonic Crystal-based Sensors | Optomechanics | VO2 Modulators | Radiation Effects |

Past Projects

Current Projects

Porous Silicon Nanoparticle Therapeutics

Porous silicon nanoparticles (PSNPs) are excellent vehicles to improve delivery of drugs owing to their high porosity, large internal surface area and biocompatibility. In particular, our group is interested in using PSNPs to enhance the delivery and therapeutic efficacy of novel nucleic-acid drugs known as peptide nucleic acids (PNA). PNA have demonstrated enormous potential in treating a wide array of diseases (including viral infections, central nervous system disorders, cardiovascular disease, and cancer), but PNA therapeutic activity is limited by poor bioavailability and low-levels of cellular uptake. Our hypothesis is that the targeted delivery and therapeutic activity of PNA can be increased by packaging PNA into PSNP delivery vehicles.

Our initial studies show that PNA therapeutics can be synthesized in situ within a PSi substrate to yield therapeutic-loaded PSNPs. These arbitrarily shaped PSNPs significantly enhanced PNA intracellular uptake, were shown to be non-toxic, and ultimately improve PNA therapeutic efficacy. In addition, we have shown that Direct Imprinting of Porous Substrates (DIPS) can be used to size and shape-engineer PSNPs. Current studies are focused on examining the effect of PSNP geometry on the enhance delivery and cellular uptake of therapeutic PNA.

(Left) Cartoon describing in situ synthesis of PNA in PSi, a process used to fabricate PNA-loaded PSNPs, or “PNA-PSNPs.” (Right) PSNPs enhance the intracellular delivery of PNA therapeutics, which otherwise do not enter the cell.

(Left) PSNPs are more biocompatible than the positive control(green), and (Right) significantly enhance PNA therapeutic activity, as measured by % increase in luciferase activity in Huh7 human hepatocarcinoma cells. (*Note “AMO” references the positive control therapeutic oligonucleotide that was delivered with Fugene 6 transfection reagent; luciferase activity measured 48hrs after treatment)

(above) Examples of possible PSNP geometries produced by DIPS.

Porous Silicon Bloch Surface and Sub-surface Waves Biosensors

PSi refractometric sensing applications have generally been size limited to molecules that diffuse into the porous matrix to cause a measurable change in effective optical thickness. Molecules approaching the average pore diameter clog the pore and hinder molecular infiltration, which significantly deteriorates the transduced signal. Hence there is a significant challenge in detecting biological entities such as viruses, bacteria, and blood cells that typically have sizes much larger than those of the pores. The PSi BSW/BSSW biosensor offers the possibility to detect both small molecules that infiltrate the pores and large molecules attached to the sensor’s surface. The BSW mode is a surface state excited within the truncated defect layer at the surface of a multilayer Bragg mirror. The novel BSSW mode is confined by a step or gradient refractive index within the multilayer and can selectively detect small molecules attached within the pores with an enhanced sensitivity (>2000 nm/RIU). The BSW and BSSW modes are each manifested as a distinct resonance peak in the reflectance spectrum, and the angular shift of each peak resulting from the light-matter interaction can be used to quantify the number of molecules attached to the sensor.

a) BSW/BSSW graphic with superimposed electric field profiles or a BSW (red), BSSW (blue), and band edge (purple) mode. b) Dispersion relation of a step index BSW/BSSW structure. The BSW appears within the band gap due to the surface defect state. The BSSW is introduced by the creation of a step refractive index profile. c) Schematic illustrating the location of large surface bound molecules and small molecules that can infiltrate the pores.

Photonic Crystal-based Sensors

Photonic crystals, periodically patterned dielectric materials, have found many uses in optical waveguiding, switching, and sensing applications, among others. Useful properties of photonic crystals include the ability to engineer photonic bandgaps at specified frequencies/wavelengths, as well as the high field confinement in defect structures. In this research project, these properties are combined with effective index material created by sub-wavelength holes fabricated inside PhC defects to create biosensors with increased surface area, and therefore, sensitivity.

Simulations were carried out using L1 and L3 photonic crystal defect designs with various multi-hole defect (MHD) structures, using FDTD software. It was found that the high quality factor (Q) associated with PhC defects was not degraded significantly by the presence of MHD. The high surface area inside the MHD increases the resonance shift compared to single surface-based sensors. Future work will involve the fabrication of these devices using standard microfabrication techniques.

(a) Dielectric constant plot of MHD simulation space, where black indicates ε = 12, white indicates ε = 1. Detailed MHD regions with effect radius (b) 0.2a, (c) 0.3a, and (d) 0.4a are also shown. The defect hole radius in all cases is 0.04a, with defect hole spacing 0.12a.

(a) Photonic bands for the photonic crystal with hole radii of 0.4a. A photonic bandgap exists for only for the TE polarization between 0.2462 and 0.4052. (b) Resonance frequency for varied defect hole dielectric constant and MHD effective radius.

Related Publications: C. Kang and S. M. Weiss, “Photonic crystal with multiple-hole defect for sensor applications,” Opt. Express 16, 18188 (2008). [Also appears in Virtual Journal for Biomedical Optics vol. 3, issue 12 (2008)] (link)

Research in Optomechanics

The research of optomechanics in our lab is to advance the understanding of light-matter interaction and gradient optical forces through the design, fabrication, and characterization of engineered photonic structures. Discovering strategic approaches for enhancing light-matter interaction and creating and controlling gradient optical forces is likely to open new avenues for enhancing the capture and detection of biomolecules, amplifying optical forces and optical sensitivity to mechanical motion, and realizing ultra-high sensitivity optical cavities that could form the basis for compact and energy efficient modulators and lasers.

(a) Dielectric constant plot of MHD simulation space, where black indicates ε = 12, white indicates ε = 1. Detailed MHD regions with effect radius (b) 0.2a, (c) 0.3a, and (d) 0.4a are also shown. The defect hole radius in all cases is 0.04a, with defect hole spacing 0.12a.

Silicon Modulators Using Metal-Insulator Switching of Vanadium Dioxide

We present an optical modulator based on a silicon ring resonator coated with vanadium-dioxide (VO2) motivated by the need for compact silicon-compatible optical switches operating at THz speeds. VO2 is a functional oxide undergoing metal-insulator transition (MIT) near 67C, with huge changes in electrical resistivity and near-infrared transmission. The MIT can be induced thermally, optically (by ultra-fast laser excitation in less than 100 fs), and possibly with electric field. VO2 is easily deposited on silicon and its ultrafast switching properties in the near-infrared can be used to tune the effective index of ring resonators in the telecommunication frequencies instead of depending on the weak electro-optic properties of silicon. The VO2-silicon hybrid ring resonator is expected to operate at speeds up to 10 THz at low Q-factor and with shorter cavity lifetimes, thus enabling compact, faster, more robust devices. We have made ring resonator structures on SOI substrates with rings varying in diameter from 3-10 um coupled to 5 mm-long waveguides at separations of 200 nm. Rings were coated with 60 nm of VO2 by pulsed laser deposition. As proof-of-concept, by switching the VO2 top layer thermally, we were able to modulate the resonance frequency of the ring to match with the predictions from our FDTD simulations.

Our recent effort has been concentrated on inducing the MIT in VO2 electrically in both linear absorption and ring resonator electro-optic modulator geometries. Gold contacts were deposited on top of the VO2 structure and a voltage applied across them. VO2 undergoes MIT at the threshold voltage and sudden jump in electrical current is observed, as well a decrease in optical transmission through the waveguide. Increasing the current though the structure after MIT occurs, increases the modulation depth making implementing analog electro-optic modulators possible.

Optical transmission of the 1.5um radius hybrid Si-VO2 ring resonator as a function of wavelength, before and after triggering the MIT with a 532nm pump laser. The lines are Lorentzian fits. Inset: IR camera images revealing vertical radiation at a fixed probe wavelength.

Schematic and SEM of the linear absorption electro-optic modulator

Example of electro-optic modulation induced by voltage pulses

Impact of High Energy Radiation on Silicon and Silicon based devices

Influence of high energy radiation on electronics has been studied extensively over the past few years, as electronics are being used in radiation environments, such as in medical equipment, nuclear reactors or aboard satellites in space. In this initiative, we examine the influence of acute high dose radiation, such as 10-keV x-rays and 662-keV gamma rays, on the silicon surface and, silicon-based optical and electro-optical devices. We found that high energy radiation exposure enhances the oxidation rate of unpassivated silicon surfaces saturating at the typical native oxide thickness of ~ 2 nm. We applied this phenomenon to a silicon based optical device: a ring resonator. Additionally, a novel amorphous silicon solar cell structure will be fabricated by the incorporation of porous amorphous silicon and it’s radiation response will be determined.

Enhanced oxidation rate observed on silicon surface due to 10-keV x-ray irradiation. The sample was exposed to dose rates ranging from 5.8 krad(SiO2)/min to 67 krad(SiO2)/min for 180 min.

Related Publications: S. Bhandaru, E. X. Zhang, D. M. Fleetwood, R. A. Reed, R. A. Weller, R. R. Harl, B. R. Rogers, and S. M. Weiss, "Accelerated oxidation of silicon due to x-ray irradiation," IEEE Trans. Nucl. Sci. 59, 781 (2012). (link)

Past Projects

Size-dependent Infiltration of Nanoscale Molecules into Nanoscale Voids

The relationship between porous silicon pore size and biomolecule size must be quantitatively determined to enable reliable infiltration and molecular binding, and to ensure the highest sensitivity detection of target biomolecules. The pore openings must be sufficiently large to allow efficient infiltration of biomolecules. When the infiltration efficiency is compromised, fewer receptor molecules are attached inside the pores and therefore fewer target species can be captured and detected. However, if the pore size is too large, then the refractive index change resulting from the incorporation of small biomolecules in the porous host material will be too small to detect and scattering losses may dominate. Moreover, choosing an oversized pore compromises the size-exclusion and filtering benefits afforded by the porous matrix. We studied the efficiency of biomolecule attachment in nanoscale porous silicon by exposing PSi membrane waveguides with three distinct average pore diameters (15 nm, 30 nm, and 60 nm) to DNA molecules with different lengths (8-bases [~1.8 nm] to 24 bases [~5.3 nm]). The figure below shows our experimental measurements of the waveguide resonance shifts after DNA attachment. The largest pores accommodate all DNA sequences investigated but the shortest sequences are more sensitively detected in the smaller pores.

Resonance angle shifts of single-layer porous silicon membrane waveguide with different pore sizes for detection of 100-\u03bcM DNA molecules of different lengths (~2.2 Å /base). For optimal performance as a sensor, the pore size must be appropriately tuned based on the size of the molecule of interest.

White Light LEDs

With the potential for much longer lifetimes and lower energy consumption as compared to current lighting technologies, the use of white light-emitting diodes (LEDs) for general lighting, an area of research known as solid state lighting, is likely to be a disruptive technology in the lighting industry in the coming years. They have the potential to be more than twice as efficient as fluorescent lighting and ten times as efficient as incandescent lighting. Also, environmental benefits such as the reduction of carbon dioxide emissions and a reduced dependence on fossil fuels for lighting can be achieved.

Thus, there has recently been a great interest in the development of white-light phosphors for solid state lighting devices, specifically for use in light emitting diodes (LEDs). We have recently demonstrated cadmium selenide (CdSe) nanocrystals with a broad, balanced emission spectrum over the visible region due to their “magic size” diameter of 1.5 nm. When encapsulated in a perfluorocyclobutyl (BP-PFCB) polymer and coated on commercial UV LEDs, these magic-sized CdSe nanocrystals act as frequency downconverters to produce light from 420-710 nm, making them an excellent material to use as a white-light phosphor for LEDs.

Thus far, we have found that the choice of encapsulant plays a significant role in the optical properties of the nanocrystal films. Some encapsulants caused a quenching of the nanocrystal emission, a modification of the white-light spectrum, and/or did not provide protection for the nanocrystals where the films became discolored and the emission intensity decreased greatly in just a few days at temperatures found in an LED junction. The BP-PFCB polymer we have focused on provides a robust and stable environment for the nanocrystals while maintaining the unique optical properties of these ultra-small CdSe nanocrystals. With the color quality of these devices in the range of those desired for general lighting, we are now focusing on increasing the efficiency of our devices by optimization of the nanocrystal concentration, UV LED wavelength, and film thickness, investigation of their scattering properties, as well as the use of dielectric thin film mirrors to increase the extraction efficiency.

Absorption and emission of white-light CdSe nanocrystals, where the dashed lines are the absorbance curves and the blue lines are the emission curves. The red spectra are the nanocrystals in a toluene solution and the blue spectra are the BP-PFCB polymer encapsulating the nanocrystals in a film. The inset shows a color photograph of a UV LED coated with encapsulated nanocrystals.

The emission from white-light CdSe nanocrystals in different encapsulants, with the highest intensity seen for the BP-PFCB polymer.

Related Publications: M. A. Schreuder, J. D. Gosnell, N. J. Smith, S. M. Weiss, and S. J. Rosenthal, “Encapsulated white-light CdSe nanocrystals as nanophosphors for solid-state lighting,” J. Mater. Chem. 18, 970-975 (2008) (link)

Direct Imprinting of Porous Substrates

Porous materials offer unique properties that have lead to great interest in utilizing them as materials for many applications ranging from drug delivery and imaging, chemical and biomolecule sensing, to catalysis, light emission, biomaterials, optoelectronics, photovoltaics and many others. However, patterning these materials, which is necessary to fabricate many device structures and particles, 3-D morphologies, arrays, gratings and a variety of important optical structures, requires the use of lithographic techniques that add both time and cost to device fabrication. Example methods used to pattern porous materials such as porous silicon include photolithographic patterning and reactive ion etching, ion implantation, UV laser ablation, localized photo-oxidation, and dry removal soft lithography. Our technique, coined "Direct imprinting of porous substrates," or DIPS, is a direct-to-device technique for mechanically deforming porous nanomaterials to produce well-defined micro- and nano structures. While repeatably producing nanoscale features (<100nm) the overall process is remarkably simple, rapid, and low-cost.

Top view SEM image of porous silicon patterned by DIPS.

Free standing porous silicon microparticles fabricated by performing DIPS with a silicon grating stamp, rotating 90 degrees and stamping again. In this image, the particles are sitting on the original stamp used to define them.