Several of the assembly processes, inorganic nanoparticles (NPs) and nanoclusters are the most attractive ones. Recently, the researches on the synthesis, characterization and applications of inorganic NPs have been increasing significantly.
The inorganic NPs have some advantages: (1) Many well-developed synthesis methods of the NPs have been proposed, which are often simple and cheap for large quantity preparation. (2) The NPs have their unique optical, electronic, and catalytic properties, which are quite different from those of their corresponding bulk materials.
For example, when the size of NP becomes smaller than its Bhor exciton radius (~6 nm for CdSe NP), the photo-excited electrons are delocalized. (3) The size of NPs, conventionally ranging from one to hundreds of nanometers, is particularly suitable
for them to serve as building blocks for the assembly of larger nanostructures and contact closely with the micro systems, like the silicon chips.
In this section, we review some significant experiments about nanodevices composed of nanomaterials or NPs. One of the methods to construct nanodevices is the self-assembly techniques, which provide a means to realize structures such as quantum dots (QDs), NPs and other electronic / optoelectronic device configurations.
Because these techniques do not rely on lithography to realize the specific nanostructures and assemblies, they can represent efficient, high throughput fabrication approaches. For self-assembled semiconductor structures, the electronic device functionality has been limited by the difficulty in achieving suitable interfaces for passivating and contacting the resulting islands or dots [15]. A patterning method of trapping and deposition of NPs in a submicron narrow gap have been developed in recently year. It demonstrated a light-emitting device, which consists of NPs trapped in the gap of lateral electrodes. The CdSe/ZnS NPs in the solvent were electrostatically trapped as a dielectric material in the gap by lateral electric field. The NPs were deposited in the gap as the solvent was evaporating. Electroluminescence from the NPs in the gap was observed when current was applied through the lateral electrodes [16]. Fig 1.5(a) shows a schematic diagram of the method. It was able to fabricate an ultra small light source smaller than the wavelength of visible light. This small light source can be applied to optical devices, such as scanning probes, integrated photonic crystals, and so on.The fabrication process of the submicron sized light source was shown in Figure 1.5(b). At first, an electrode structure on p-type SOI was made with DRIE (The Alcatel A601E), and then a submicron gap was fabricated by cutting the electrode structure with Focused Ion Beam (FIB) that can enable us to fabricate a submicron narrow gap easily. Next, the wafer was immersed in NPs solution of toluene solvent, and applied the voltage to the gap between the lateral
electrodes to trap NPs. At last, wafer was annealed at 400℃ to remove excess organic molecules such as toluene and TOPO. After annealing, junctions between p-type Si and CdSe/ZnS NPs were built [16].
The synthesis of quasi-one-dimensional (1D) nanostructures has been developed, many of which have interesting electronic, optical, and chemical sensor properties that derive from size, composition, and shape. One-component systems are now quite common, but there are few examples of methods for synthesizing multi-component 1D materials composed of organic and inorganic materials. The hybrid multi-component (i.e. organic-inorganic) nanorods that have either diode or resistor properties has been proposed in [17]. In a typical experiment, the synthesis of segmented metal-polymer nanorods by electrochemical deposition of gold into alumina templates, followed by electrochemical polymerization of pyrrole (Ppy).
During the electrodeposition process, it can control the length of each block by monitoring the charge. Other metals (e.g. Ag and Cd) with low work functions and inorganic semiconductors (e.g. CdSe), also can be deposited on top of the polymer block and polyaniline can be used in place of polypyrrole. This allows one to prepare multi-component rod structures with tailorable electronic properties that derive from the choice of the individual compositional blocks. For the Au portions of the nanostructure (contacts 1-2 and 3-4) exhibit linear I-V characteristics and bulk metallic behavior at room temperature as shown in Fig. 1.7(A), and demonstrate Ohmic behavior. For the Au-Ppy-Au system, one can see dark Ppy domains sandwiched between two bright segments of gold as shown in Fig. 1.6(A).
Significantly, Fig. 1.7(B) shows the I-V measurements across the Ppy block of the Au-Ppy-Au nanorod (2-3 and 1-4 contacts) also exhibit a highly reproducible, linear response at room temperature but nonlinear behavior at low temperature (<175 K),
conductivities provide two important observations. First, the conductivity of the polymer block at room temperature is 6 orders of magnitude lower than the metallic blocks, and all data are consistent with Ohmic contact between the Ppy-Au junctions.
Second, the I-V response for the Au-Ppy-Au nanorod becomes slightly nonlinear as the temperature decreases [Fig. 1.7(B)]. Since the Ppy for the nanostructures discussed herein were generated by oxidative polymerization, they are p-type semiconductor. Continuously, four-segment nanorods (Au-Ppy-Cd-Au) also can be prepared via an analogous procedure [Fig. 1.6(B)]. I-V measurements on devices constructed from single Au-Ppy-Cd-Au rods exhibit “diode” behavior at room temperature as shown in Fig. 1.7(C), and the typical response is asymmetric and non-Ohmic. In the forward bias, there is a positive voltage on the Au block adjacent to the Ppy and negative potential on the Au block interfaced with the Cd block.
Therefore, holes move from the Ppy block to the Cd block during the forward bias. In reverse bias, current does not flow until the bias overcomes the breakdown potential (-0.61 V). The turn-on voltage for these diode nanorods is approximately 0.15V. The I-V characteristics of the Au-Ppy-Cd-Au nanorods at room temperature suggest that an Schottky-like junction is formed at Ppy/Cd due to the difference in work functions of the two materials and an Ohmic junction at the Ppy/Au interface due to the similarity in work functions for the two materials. It is a powerful method for deliberately producing structures with desirable electrical properties with a straightforward synthetic procedure that offers a high degree of reproducibility [17].
These structures could be useful for a wide range of electronic and sensor devices.
Now, we will introduce the state of current and coming solar photovoltaic technologies and their further development. The emphasis is on R&D advances and cell and module performances, with indications of the limitations and strengths of crystalline (Si and GaAs) and thin film (a-Si:H, Si, Cu(In,Ga)(Se,S) , CdTe). The
contributions and technological pathways for now and near-term technologies (silicon, III–V, and thin films) and status and forecasts for next-next generation photovoltaic (organics, nanotechnologies, multi-multiple junctions) are evaluated. Recent advances in concentrators, new directions for thin films, and materials/device technology issues are discussed in terms of technology evolution and progress. Insights to technical and other investments needed to tip photovoltaic to its next level of contribution as a significant clean-energy partner in the world energy portfolio [18]. The research progress over the past 25–30 years has been substantial and steady, as shown in Fig.
1.8. Photovoltaic is poised at what may be its most critical tipping point; the one that will cause this technology to “spread like wildfire” as it finally becomes a major part of our world’s energy portfolio.
Recently, in order to provide innovative strategies for designing next generation energy conversion devices, people efforts to design ordered assemblies of semiconductor and metal NPs as well as carbon nanostructures. Renewable energy such as solar radiation is ideal to meet the projected demand but requires new initiatives to harvest incident photons with higher efficiency, for example, by employing nanostructured semiconductors and molecular assemblies. Dye sensitization of mesoscopic TiO2 has been widely used in this context. Power conversion efficiencies up to 11% have been achieved for such photochemical solar cells. Semiconductors such as CdS, PbS, Bi2S3, CdSe, and InP, which absorb light in the visible, can serve as sensitizers as they are able to transfer electrons to large band gap semiconductors such as TiO2 or SnO2. CdSe quantum dots (QDs) have been assembled onto mesoscopic TiO2 films by using bifunctional surface modifiers (SH-R-COOH) [19]. During visible light excitation, CdSe QDs inject electrons into TiO2 nanocrystallites. The injected charge carriers in a CdSe-modified TiO2 film can
composite, when employed as a photoanode in a photoelectrochemical cell, exhibits a power conversion efficiency of 12%. Significant loss of electrons occurs due to scattering as well as charge recombination at TiO2/CdSe interfaces and internal TiO2
grain boundaries. Fig. 1.9 shows the assembled TiO2 and CdSe NPs using bifunctional surface modifiers of the type HS-R-COOH. Fig. 1.10 shows the sequence of steps followed.
One approach to facilitating electron transport in nanostructured semiconductor films involves applying a positive bias to the working electrode. The photocurrent generated at different applied potentials for OTE/TiO2 and OTE/TiO2/CdSe electrodes is shown in Fig. 1.11. Excitation of TiO2 and TiO2/CdSe films was carried out using light with wavelengths greater than 300 and 400 nm, respectively. Both films show anodic photocurrents when subjected to band gap excitation. The observed photocurrents increase as the potential is swept toward positive values. The potential at which we observe zero current is a measure of the flat band potential and reflects the maximum attainable open-circuit voltage (Voc). We observe zero current at potentials of -0.78 and -0.88 V vs. SCE for TiO2 and TiO2/CdSe films, respectively.
The 100 mV shift represents the improved energetic of the TiO2/CdSe films and shows theadvantage of using composite nanostructures for boosting Voc [19].
1.3 Motivation
As we have discussed previously, interactive forces between molecules, such as hydrogen bonds, Van der Waal force, and coulombic force, are also effective for NPs and play an important role in the assembly process. The researches on the synthesis of organic and inorganic NPs have been increased significantly. Among the semiconductor NPs, CdSe QDs or QDs is the most suitable for harvesting light energy
in the visible region of the solar spectrum. So many researchers always choose the CdSe QDs or QDs to realize the nanodevices, optical devices, and solar-like devices.
Recently, the photo-sensing nanodevice composed of negative-charged Au and positive-charged Tyramine-CdSe QDs has been developed and proposed. This functional nanodevice composed of inorganic NPs directly on the surface of silicon chip is the simplest and most effective process and without damage of the circuits in silicon chip. However, the method of Tyramine modification on CdSe QDs will seriously damage the optical and electrical properties of CdSe NPs. Therefore, it is important to develop more efficient methods. In our works, we propose another modify method of CdSe QDs and follow the ｀dipping-and-washing＇ process to improve the performance of Au / CdSe nanodevice, continuously, we also construct another nanodevice composed of CdSe-modified QDs based on“self-assembly technology＂.
Meanwhile, to correctly define the pattern of the QDs and NPs, lift-off process was utilized for this purpose. Finally, three dimension nanodevice model is simulated by HSPICE. It successfully explains the phenomenon of the characteristics of the nanodevice. The power conversion efficiency can achieve 40% based in our ideal interference on the basis of the 3-D nanodevice model.
1.4 THESIS ORGANIZATION
The background has been introduced in section 1.1, including the basic concepts, the trend of nanotechnology development in the world, the synthesis methods of Au and CdSe QDs, and several of assembly methods of NPs on silicon substrate. Then, we have some reviews on the most representative experiments about nanodevices
based on nanomaterials, QDs, and NPs in section 1.2. At last, the motivations of this work and thesis organization will be proposed in section 1.3 and section 1.4.
In chapter 2, the fabrication technology will be discussed, including the process flow and the nanodevice structure design concepts. In section 2.1 and 2.2, the synthesis of Au NPs and CdSe QDs will be introduced. Then, the physical characteristics of the nanodevices will be demonstrated. In section 2.4, some experiments of the environment factors were tested to optimize the reaction conditions. In section 2.5, the electrode process with the lift-off technology will be proposed to solve the unexpected NPs and QDs. In section 2.6, the self-assembly technology of the nanodevice process is announced.
In chapter 3, experimental results will be showed and discussed. First, the measurement environment is introduced in the section 3.1. Secondly, the nanostructure physical characteristics would be demonstrated, for example, SEM view and absorption / emission Spectra. Thirdly, different layer Au / CdSe (AET-CdSe and PDDA-CdSe) Nanodevice would be measured. Then, the results would be showed and discussed in section 3.3. Next, Nanodevice solar cell efficiency would be estimated in section 3.5. Finally, using HSPICE software to construct the nanodevice model was executed in section 3.6. The simulation result fits the nanodevice measurement results, and it also could explain the electrical characteristics of the different dimension nanodevice.
In chapter 4, a linear regulator is designed to target the low voltage conditions and integrated with the CdSe / Au nanoparticle solar cell. The relevant analysis is introduced to design the system, and the TSMC 2P4M CMOS 0.35um technology is used to implement the linear regulator. Finally, the measurement result of the linear regulator chip is showed and discussed.
In chapter 5, the conclusions and future works are given in section 5.1 and 5.2.
Figure 1.1 Chemistry is the central science for further applications such as materials science and biotechnology. The combination of advanced materials and tailored biomolecules will produce the future nanodevices [1].
Figure 1.2 A gap currently exists in the engineering of small-scale devices. The top-down processes will have their limit below 100 nm, and the bottom-up processes will also have a limit at 2~5 nm. The gap will be filled by nanoclusters and biomolecules [1].
Figure 1.3 The TEM images of SiO2@Au NP clusters synthesized at (A) pH=8.4, (B) pH=8.6, (C) pH=10.2, (D) pH=11.1. The scale bar for all micrographs is 200 nm [2].
Figure 1.4 The TEM images of SiO2@CdSe NP clusters synthesized at (A) pH=6.8, (B) pH=7.2, (C) pH=10.2, (D) pH=11.1. The scale bar for all micrographs is 100 nm [2].
(a)
(b)
Figure 1.5 (a) The schematic diagram of trapping NPs in a submicron narrow gap (5 μm * 5 μm * 1 μm) and a submicron sized light source. (b) The fabrication process of a submicron sized light source based on SOI and CdSe QDs [16].
Figure 1.6 (A) Optical microscope image of Au-Ppy-Au rods. (B) Optical microscope image of Au-Ppy-Cd-Au rods. The lower left inset shows the corresponding FESEM image [17].
Figure 1.7 The measurement results of I-V characteristics. (A) For the gold blocks (1-2, 3-4) within a single nanorod at room temperature. Inset shows the optical microscope image (1000 magnification) of a single Au-Ppy-Au rod on microelectrodes. (B) Temperature-dependent I-V curves for measurements across electrodes 2 and 3. (C) For a single Au-Ppy-Cd-Au rod at room temperature [17].
Figure 1.8 The efficiency evolution of best research cells by several of technology types. This table identifies those cells that have been measured under standard conditions and confirmed at one of the word’s accepted centers for standard solar-cell measurements [18].
Figure 1.9 (a) Linking CdSe QDs to TiO2 particles with bifunctional surface modifier (HS-R-COOH); (b) Light harvesting assembly composed of TiO2 film functionalized with CdSe QDs on Optically Transparent Electrode (OTE) [19]. (Not to scale)
Figure 1.10 The sequence of steps for linking CdSe QDs to TiO2 surface with a bifunctional surface modifier [19].
Figure 1.11 I-V characteristics of (a) OTE / TiO2 and (b) OTE / TiO2 / MPA /CdSe films. The filtered lights allowed excitation of TiO2 and CdSe films at wavelengths greater than 300 and 400 nm, respectively [19].