Projects Synthesis Materials Fabrication Devices Characterization Theory
Hybrid Solar Cell   ReRAM  
Heterojunction FET   Nanomanipulator  
nanowire p-n junction   DNA nanomachine  
Addressable nanobody array  
Hybrid Solar Cell
Solar cells are based on the photovoltaic effect which is defined as the generation of an electrical current in a circuit containing a photosensitive device when the device is illuminated by visible or nonvisible light. Commercial solar cells are in their third generation of development since the patent of modern solar cells in 1946. Most cells on the market though are still first or second generation, consisting of large-area crystalline (or poly-crystalline) silicon or layers of silicon, which are doped in a way that leads to p-n junction diodes.

Third generation devices are defined as semiconductor devices that do not rely on the traditional p-n junction to separate photogenerated charge carriers and their designs vary greatly. In recent years the incorporation of different materials other than doped silicon has become more common because in fact material properties (not the photovoltaic effect), such as band gap and mobility, determine the photon absorption and conversion to electricity. In general, the more narrow the band gap the longer wavelength of light can be absorbed by the material. Small band gaps, close to .8eV, absorb infrared light. This proposal deals with a hybrid design which incorporates both conducting polymers and inorganic nanorod and nanostructured materials. The band gap of the inorganic layer can be systematically altered in order to achieve absorption of a wide spectrum of light including the depths of the infrared region and short wavelengths in the UV. This approach, the process of controlling or altering the band gap of a material by controlling the composition of certain semiconductor alloys, is known as bandgap engineering.

The inorganic materials are mixed with conducting polymer and incorporated into a novel multilayer design on a transparent conducting substrate. Confirmation of the functionality of the device requires electroluminence, x-ray diffraction, TEM, photoconductivity and other studies.

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ReRAM
The increased usage of mobile devices like multi-functional cellular phones, MP3 players, digital cameras, USB memory, etc. has lead to the drastic increase in demand for nonvolatile memory. A popular usage for nonvolatile memory is a floating gate flash memory, which has storage limitations of 16 Gbit to 32 Gbit. Maximum storage is dependent on the materials and fabrication methods. In order to overcome this limit, different kinds of materials are being incorporated into new types of memory systems. These new systems are more suitable to miniaturization and include the following - RRAM (phase change random access memory), NFGM (nano-floating gate memory), PoRAM (polymer random access memory), ReRAM (resistance random access memory), FRAM (ferroelectric RAM), MRAM (magnetic ram), RRAM (phase change random access memory), NFGM (nano-floating gate memory), PoRAM (polymer random access memory), ReRAM (resistance random access memory), and FRAM (ferroelectric RAM), MRAM (magnetic ram).

The current focus in the Nanoscience Laboratory is on ReRAM, which has fast write/erase time(10 to 100 nsec) and good thermal stability. ReRAM is made up of one transistor and one resistor as base components. If a negative voltage is applied to the resistor (which would be due to drain current through the transistor), the resistance in the resistor increases, and this increased resistance is sustained. Therefore current flow is low until a positive voltage is applied to the resistor. Figure 2 shows such behavior, which in essence is the way this type memory device functions.

One of the base materials of ReRAM is transition metal oxide (TMO). In this proposal, the idea is to use a TMO nanowire to achieve writing faster than 100 nsec, high write/erase cycle ratio over 106 and to prevent degradation at high temperature. A candidate material is currently NiO. In Figure 1 one can see that NiO is chosen because it posseses the correct resistance switching property that is needed in ReRAM.

The final goal of this project is to develop and fabricate a functional ReRAM device based on vertically aligned TMO nanowires. In addition the aim is to investigate the details of the switching mechanism of ReRAM using TMO.

References:
1. Min Gyu Kim et al, J. Jap. appl. phys. 44 L1301 (2005)
2. MJR, Inoue and Sanchez, Phy. Rev. Letters., 92, 178302 (2004)
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Heterojunction FET
A hetero junction FET device can be fabricated by using a single nanowire by half masking technique as seen in figure 1(left). Also an abrupt cross-junction FET device using each individual nanowire, as is shown in figure 1(right), can be made. Various properties of nanowires can be studied with these FET devices, for example by using Raman spectroscopy, XRD, PL, simulation of band diagram by the electrical properties of temperature dependence and so on.


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Nanomanipulator
We have fabricated various types of nano-probes based on a single carbon nanotube using nano-manipulators inside the field emission scanning electron microscope (FESEM). We can attach an individual carbon nanotube to an etched tungsten tip and other various scanning probe microscope (SPM) tips using an electron beam induced deposition (EBID) method and Joule heating method. The probes fabricated by the EBID method have a large contact resistance. One the other hand, the Joule heating method yields good electrical contact. Thus we can successfully make tips for a wide range of applications. For example some devices using these tips are the atomic force microscope (AFM), electrical force microscope (EFM), kelvin force microscope (KFM) tips, field emission source and biosensors.

Presently we have a future plan to use our expertise in preparing such tips for the fabrication of a low energy e-beam source. There are many advantages to using a low energy e-beam source - power saving, easy doping of certain materials, modification of individual nano materials and so on.
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nanowire p-n junction
This p-n junction diode uses p-type and n-type metal oxide nanowires. Two individual metal oxide nanowires coupled in a junction, similar as shown in the figure below where the CNT is replaced with a nanowire, can be characterized electrically. The hope is that interesting data concerning the gating effect of each material will be revealed in addition to realizing rectifying behavior from the junction-interface and photo current (wavelength dependence). In our lab, we currently grow both p-type (NiO, CuO and doped ZnO) and n-type nanowires (pure ZnO, VO2, V2O5).

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DNA nanomachine
DNA are proving to be a versatile building material in nanoscience and nanotechnology because of the structural rigidity of duplex DNA at the sub-50 nm length scale, and its versatile chemistry and biology which allows functionalization and manipulation. We recently reported the first reversible operation of a proton-fuelled DNA nanomachine immobilized on the defined surface locations. A single-stranded motor DNA, modified with a thiol at 5กฏ and a fluorophore at 3กฏ, was immobilized in an array fashion onto defined locations on a gold surface patterned by micro-contact printing. This DNA underwent reversible conformational changes, between a closed 4-stranded and an open single-stranded (forming duplex when hybridized with a complementary strand) structure as the solution pH was switched between 4.5 and 9.3. This led to a reversible change in the separation distance between the fluorophore and the gold surface, producing different levels of fluorescence quenching (hence fluorescence intensity), which was detected by scanning confocal microscopy. The reversible conformational switching was further verified by monitoring the thickness changes of the DNA layer by atomic force microscopy. When combined with other sensor platform technology, this can serve as powerful and flexible sensor devices.
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Addressable nanobody array
There is a considerable interest at present in positioning functional biomolecules at defined positions at surfaces to produce biological arrays for analysis or to assemble complex and biologically functional structures using molecular recognition between different bio- molecules. Key issues for any method are maintaining the functionality of the biomolecules, the smallest feature or spot size that can be obtained, the density of features possible, the ability to address a specific feature on the surface and to deposit different molecules for local assembly of a structure. We use a nanofabricated surface, which is functionalized with distinct chemical functionalities at different positions, so the feature size is controlled by the surface rather than the physical processes taking place during delivery. The nanofabricated structure was produced using a 30 KV kV Ga ion focussed ion beam (FIB) microscope. The structure has similarities to that used by Webb and co-workers for single molecule detection and consists of regularly spaced holes but produced within a thin film of gold (50 nm thick). This allows us to observe the position of a nanopipette which is held in solution over the holes through the gold by optical microscopy to address individual holes. The bulk of the gold surface was functionalized with a self-assembled monolayer (SAM) terminated with hexa(ethylene glycol) (EG6) groups to resist non-specific adsorption while the holes were functionalized with a SAM terminated with carboxylic acid groups to facilitate the attachment of biomolecules. Biomolecules in solution or delivered from the nanopipette thus are adsorbed preferentially into the holes. The feature size of the deposited biomolecules is therefore determined by the size of the pre-nanofabricated holes. Furthermore, this structure confines the detection volume for fluorescence spectroscopy to the volume inside the hole because of the optical properties of the gold. We show that we can fill these nanofabricated holes with functional proteins. In addition, we can also locally deliver functional antibodies into specific holes to address them individually, and detect their bindings by fluorescence.
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