The Synthesis of Functional Nanostructures Using the Ink Jet Method

Polymethine dyes are unique organic colorants with an inherent ability to associate in structured aggregates. These aggregates exhibit electronic behavior that can be leveraged in device manufacture. Research results will be presented that furthers the goal of using the ink jet technique as a method of producing a highly uniform and predictable aggregate structure capable of charge separation functionality. Introduction The printing industry is enjoying a rebirth with the growing need to produce smaller electronic devices. Gravure, flexography and digital printing methods, such as ink jet printing, have been used to produce high quality electronic devices and sensors on a production scale [1]. Printed radio frequency identification (RFID) circuits, for example, have been identified as the 21st century version of the packaging barcode [2]. Active devices, however, are going to require power. Therefore, the RIT Print Science Laboratory focuses on using the printing method as a means to produce efficient power sources for emerging nano-devices. Photovoltaic devices would be the power source of choice given their portability and renewable attributes. The generalized design of a photovoltaic cell is illustrated in Figure 1. Figure 1. Photovoltaic Device Dye-sensitized solar cells, such as the Graetzel cell, have been demonstrated, but often suffer from low conversion efficiency [3]. Alternative dye-based designs would involve the fabrication of a charge separation layer using the inherent electronic properties of the dye material rather than relying on electrochemical processes. We choose to fabricate a charge separation layer leveraging the delocalized π electronic character of the polymethine dye (PMD) aggregates. PMD aggregate properties have been extensively studied and reported in the literature [4,5]. These aggregates have also been used as the charge separation layer in photovoltaic devices [6,7] with limited success. We believe that one reason, for the limited efficiencies, results from the fabrication method. In most cases, dye aggregates are precipitated onto a TiO2 electrode surface using bulk phase dye solutions. This would result in an inhomogeneous aggregate distribution of varied structure and electronic character. Our approach is to control the synthesis, and therefore the structure of the PMD aggregate, using a two-step ink jet method. In this method, an aggregate of uniform size and structure can be crystallized via the addition of an aggregate promoter. Experimental Approach The optimum dye and promoter concentration will be determined using a two-nozzel ink jet printer developed in our laboratory (see Figure 2). Figure 2. Two-nozzel Ink Jet Printer This printer is designed using two high speed solenoid values capable of sub-microliter dispensing by controlling both head pressure (P, compressed nitrogen gas) and electronic actuation waveform (electronic timing circuit). By adjusting these parameters, we can modulate both the spatial and temporal dependence of aggregate formation with dye concentration. We have selected two dye candidates in our initial survey phase of this project. 4,5,4',5'-Dibenzo-3,3'-diethyl-9-methylthiacarbocyanine bromide (DTCB) and 1,1’-Diethyl-2,2’carbocyanine chloride (DCC) were chosen for their propensity to form J and H type aggregates in the presence of a promoter. DTCB was prepared at a 250μM concentration in distilled water. The DTCB aggregate promoter was sodium chloride prepared at a 23μΜ stock concentration. DCC was prepared at a 10mM concentration in ethanol. The DCC aggregate promoter was 0.2M sodium dodecyl sulfate (SDS) in distilled water. Solution spectral measurements were obtained using a Cary 100 Spectrophotometer configured with a 1nm bandpass. Reflective spectral measurements were obtained using an Edmund Scientific bifurcated fiber optic collection system integrated onto a Conductive Electrode Charge Separation Layer Transparent Conductive Electrode BRC111A CCD array spectrometer. The effective spectral resolution of this system is one nanometer. Results DTCB J-aggregate formation, in the presence of increasing NaCl concentration, has been well documented in the literature. Figure 3A illustrates the spectral differences between the monomer and J-aggregate form of DTCB. We found, however, that there was a significant temporal dependence on aggregate formation that may be linked to the solvation rate of reactant species. Figure 3. 250mM DTCB in water with (A) increasing concentration of NaCl promoter added dropwise into three milliliters of dye solution, and (B) reflective transmission measurements of a printed dot in the order promoter followed by dye solution. The dry promoter curve corresponds to allowing a one hour time delay between overprinting sequence to allow for sufficient