Anisotropic Polythiophene Thin Films for Charge Transport Studies
- L. Jimison

Semicrystalline polymer films consist of crystalline lamellar regions separated by disordered grain boundaries. Using a method of directional crystallization to create highly anisotropic thin films of poly(3-hexylthiophene) on silicon substrates, we have controlled the orientation and placement of the grain boundaries. The microstructure of these films has been characterized with Polarized Light Microscopy and Atomic Force Microscopy.

Figure 1 a) Shown here is a film of oriented P3HT fibers on silicon. From the behavior with polarized light, we know that the molecules are oriented such that the chain axis is parallel to the macroscopic fiber axis. b) The AFM phase mode image reveals the lamellar stacking along the fibers. The lamellar repeat distance is approximately the length of an extended chain.

We have also conducted diffraction experiments at Stanford Synchrotron Radiation Laboratory (SSRL), which include Grazing Incidence X-Ray Scattering (GIXS) with both a two-dimensional area detector and a high resolution point detector, and specular diffraction with a high resolution point detector. Top contacts are deposited on the oriented films on highly doped silicon to complete the Thin Film Transistor (TFT) geometry. Both temperature dependent and gate voltage dependent mobilities are extracted from IV curves. By fitting this data to the Mobility Edge Model (see Salleo, PR B 70 (2004)), we can get a better understanding of how trap states in grain boundaries effect charge transport properties in these semiconducting polymer films.

Figure 2 Polythiophene films are semicrystalline. A typical thick film grazing incidence x-ray scattering (GIXS) image is shown above in the top right. The rings indicate no texture, but instead simply a randomly oriented polycrystalline film. A typical thin film GIXS pattern is shown on the bottom right, where the (h00) peaks lie along the qz axis and therefore out of the plane of the substrate, and the pi-stacking direction lies in the plane of the substrate, with the (010) on the qxy axis. The oriented films in this study have an unusual texture, see the GIXS pattern on the top left. Many of the crystallites are oriented such that the pi-stacking direction lies out of plane with respect to the substrate.

Patterning polymeric semiconductor circuits and nanostructures by surface-directed phase separation
- L. Jimison, J. Rivnay

Printing electronic circuits on flexible substrates at low cost is widely held to be a truly disruptive technology. As a consequence, there has been much recent interest in developing electrically functional materials in liquid form. Conjugated polymers are suitable semiconducting materials for printed electronics because they can be dissolved in organic solvents. The goal of this technology is to make manufacturing of electronic devices akin to roll-to-roll color printing, where the electronic materials play the role of the color inks. However, a long-standing promise of solution-processed electronics is the prospect that a blend of materials in solution can self-organize to form devices and circuits. Here we show the spontaneous self-organization of a semiconductor/insulator mixture into an array of isolated and encapsulated polymeric thin-film-transistors (TFTs). We demonstrated the self-assembly of a 64 polymer TFT array with a yield higher than 95%. The individual devices performed identically to polymeric TFTs made by conventional means and their off-current is typically smaller than 1 pA, indicating a high degree of device isolation. This work constitutes the first step towards self-assembly of multi-layer, multi-component circuits out of solution. By patterning different surface functionalities we envisage that multi-component solutions can be brought to selectively phase separate on designated areas of a substrate, thereby achieving self-assembly of entire functional systems in a single solution-based process.

This process promises to allow the patterning of polymeric thin films without using photolithography, which is known to damage these delicate materials. We are exploring the use of this technique to self-assemble electronic circuits. Moreover, we plan on studying the effect of lateral confinement on the separation process with the goal in mind to pattern polymeric nanostructures for fundamental studies of charge transport in polymeric semiconductors.

Patterned phase separation of a PQT-12/PMMA blend (a). PQT-12, a polymeric semiconductor, is the patterned purple film. SEM of the patterned PQT-12 film after stripping PMMA (b). The arrows show that sub-micron features can be patterned by phase separation. Structure of a phase separated thin film showing the self-assembly of isolated TFTs.

Nanostructured ZnO for solution-based transparent electrodes
- L. Goris, R. Noriega, S. Padke, S. Mehra

Transparent conducting oxides have gained great industrial importance. The popularity of flat-screen technology (television, computer, phones, i-pods,…) and the recent greentech boom -which put photovoltaic modules at the forefront of the quest for efficient renewable energy sources- have greatly increased the use of Indium Tin Oxide (ITO). As a result, the price of In has recently surpassed that of silver. It is widely believed that the price volatility of In will become an extremely detrimental factor in the fabrication cost of photovoltaic cells. We are currently seeking to replace ITO with a low-cost materials set that is compatible with large-area, low-cost processing techniques, such as printing.

Zinc based oxides, can be easily processed from solution in order to produce thin, homogeneous films that can be used as electrodes in the aforementioned applications. Because of the abundance and low toxicity of Zn, these materials are a very attractive replacement for ITO. Pure ZnO is an intrinsic semiconductor but it can be made conductive by doping it with Al, Ga or F. Several methods have been described to form polycrystalline, highly doped ZnO, the most successful being Chemically Vapor Deposition (CVD), sputtering and spray pyrolysis of Zn based salts. These techniques, however, have many disadvantages, such as the use of expensive vacuum equipment and high processing temperatures, which makes them incompatible with flexible substrates. Moreover, effective doping can be problematic due to the high reactivity of Al with residual oxygen, preventing its uniform incorporation into the ZnO lattice.

In our group, we use a colloidal synthesis in an organic solvent for the synthesis of ZnO nanostructures (particles, rods and wires). Based on previous work by Yang et al.,1 we produced nanoparticles and nanowires (W~100 nm, L~1-2 μm). The size and aspect ratio of the nanostructures depends on the concentration of the Zn salt and the presence of surfactants (Fig 1).

Fig 1: Left: ZnO nanoparticle synthesis, without surfactant Right: ZnO nanowire synthesis, with surfactant

These nanostructured ZnO powders can be easily resuspended in ethanol and spincoated into very uniform films that are both diffusing and transparent (Fig 2).

Fig 2: Nanostructured ZnO “ink” (a). Spin-coating of the ZnO “ink” on a glass slide produces a uniform film with a milky-white appearance (b). The spin-coated film is highly diffusing yet transparent (insert).

By decomposing Al salts during the ZnO nanowire growth, we try to actively dope the wires and achieve a low sheet resistance. Studies on the effect of temperature and relative salt concentration are being performed to optimize the doping process. To understand the growth kinetics of the process, we plan on starting a study where the growth is monitored in-situ under synchrotron radiation. This work is performed in collaboration with Dr. M. Toney and the Stanford Synchroton Radiation Laboratory (SSRL).

Quantum Dot Formation in SiOx through Femtosecond Laser Annealing
- W. Mustafeez

Figure 1. Waveguide writing in Silicon Rich Silicon Oxide through high repetition rate femtosecond lasers: focal volume with multiphoton absorption occuring below the surface of transparent material.

Femtosecond Lasers through their high peak electric fields offer excellent versatility especially in nonlinear process driven applications. They find usage in a wide array of areas from materials processing to driving free electron lasers. In case of materials processing, femtosecond lasers offer nanometer scale precision machining with minimal spread of damage.

This is possible due to the ultrashort pulse lengths which are below the timescale of many fundamental processes such as carrier phonon scattering. Hence for surface processing fs lasers are an excellent tool. However due to the high peak electric field intensities they also open up the possibility of subsurface material processing through multi-photon absorption. Using high numerical aperture objectives it’s possible to perform phase transformation or damage to very small focal volumes even with unamplified nanojoule pulse energies. The field of femtosecond laser written waveguides and other structures in various transparent materials has received wide interest with possible applications in on-chip interconnects, photonic crystals etc.

Within the motivation of on-chip photonics, work on the optical properties of silicon quantum dots or nanocrystals have been going on since the early 90s. It has been shown that the emission properties of nanometer scaled silicon crystals are much more favorable in contrast to bulk for development of light sources. In fact in 2000 (Pavesi et al.) the possibility of optical gain in silicon nanocrystals was shown for the first time. Most of these studies either employ ion beam implanted Si in a matrix of SiO2/SiN or low temperature Plasma Enhanced Chemical Vapour Deposition (PECVD) deposited non-stoichiometric SiO2/SiN followed by high temperature anneal for over an hour.

Figure 2. Subsurface phase transformation in 2µm thick SRSO using high rep-rate oscillator. Inset shows visible PL from oven annealed SRSO.

Our group is working on using a high repetition rate laser oscillator for creating waveguides in PECVD deposited SiOx. The high repetition rate ensures that the time between pulses is significantly below the thermal diffusion time and hence the successive pulses can cause an energy buildup, rapid annealing and finally precipitation of silicon nanocrystals, embedded in a waveguide. Since we are using a laser wavelength well below the band gap of SiO2 and a high NA objective we can precisely control the depth and in plane position of the energy disposition.

We use unannealed SiOx films for the processing. Figure 2 shows a diascopic microscopy image of a laser processed waveguide, approximately 3µm wide and 100µm long. This experiment was performed with 800nm Ti:Sapphire laser oscillator at 75MHz repetition rate and 5.3nJ of energy per pulse. The translation speed was about 200µm/sec. We used a 1.2NA water immersion objective to focus the beam onto the SRSO film. A standard coverslip was placed above the sample. The feature shown is not visible in reflected light microscopy hence is subsurface. Development of this process will open up possibilities in spatially precise placement of Si-NC which may find use in studying Si quantum dots more closely as well enable practical applications such as using the Si-NC sandwiched between metals for coupling light onto MIM plasmonic waveguides etc.