This project aims to develop a synthesis of ultra-long cadmium selenide (CdSe) nanowires that are suitable for optoelectronic applications. A goal of this work is to learn the degree to which the spectroscopic properties of polycrystalline CdSe nanowires resemble that of single crystalline CdSe nanorods. CdSe nanowires were synthesized using our Electrochemical Step Edge Decoration method by nucleating CdSe growth on Se “seed” nanowires (Fig. 1). Stoichiometric CdSe is obtained using a cyclic electrodeposition/stripping method (Fig 2) that continuously and selectively removes excess cadmium metal from the deposited CdSe. In the final step (Fig 1) CdSe nanowires are heated in H 2 S thereby forming a “cap” of CdS (E BG = 2.42 eV). The CdSe nanowires prepared by this method show photoluminescence near the 1.74 eV bandedge expected for CdSe (Fig 3a). Adding the CdS capping layer dramatically increases the band-edge photoluminescence from these nanowires. A key point is that no red-shifted trap state emission, and no CdS emission, is seen from these nanowires. Electrodeposition of Luminescent Polycrystalline Cadmium Selenide Nanowires Reginald M. Penner DMR Figure 1. Synthesis of CdS and CdS-capped CdSe nanowires. Figure 2. a) CdSe nanowires are obtained using a cyclic electrodeposition/stripping scheme in which excess Cd in the electrodeposited CdSe is removed on each cycle. b) The diameter of the CdSe nanowires obtained using this method increases with the number of synthesis cycles. c) SEM image showing CdSe nanowires that were ≈30 nm in diameter. Figure 3. a) Photoluminescence (PL) spectra for CdSe nanowires and CdS-capped CdSe nanowires ( ex = 257 nm). No emission for CdS at 2.42 eV is observed in these spectra, but CdSe emission intensity is increased by 15x for the CdS- capped nanowires. (b) White light optical micrograph (50 x 50 µm 2 ) of CdSe/CdS nanowires (dark lines) on graphite. (c) Fluorescence micrograph ( ex = 257 nm) of the same location shown in (b) showing bright orange PL from these nanowires.

“ZT” is the dimensionless figure-of-merit that gauges the efficiency with which a material converts thermal energy to electrical energy. According to theory, reducing the dimensionality of a thermoelectric material like Bi 2 Te 3 - from bulk (3D) to films (2D) to nanowires (1D) should boost ZT, but there has been no experimental confirmation of this result for nanowires and, in fact, the highest ZT seen for Bi 2 Te 3 nanowires has been 0.012, much lower than the ZT=1.0 measured for bulk Bi 2 Te 3. We use step edges on graphite to template the electrodeposition of Bi 2 Te 3 nanowires that are up to 1 mm in total length. During synthesis, the potential of the graphite surface is first scanned to produce stoichiometric Bi 2 Te 3 nanowires. These nanowires have a diameter that increases with the number of synthesis cycles, as shown in Fig. 2a,b,c. To prepare wires smaller than 100 nm, we then electrooxidize larger Bi 2 Te 3 nanowires, as shown in Fig. 2 d,e,f. Bi 2 Te 3 nanowires as small as 30 nm in diameter can be produced using this combination. These Bi 2 Te 3 nanowires were used to generate electrical power by transferring ensembles of these nanowires onto insulators (Fig 3). We measured ZT values of up to 0.85 on ≈130 nm diameter Bi 2 Te 3 nanowires. This result suggests that the polycrystallinity of these Bi 2 Te 3 nanowires does not impede their thermoelectric performance. Bismuth Telluride (Bi 2 Te 3 ) Nanowires for Efficient Thermoelectric Power Generation Reginald M. Penner DMR Figure 3. a) Using an evaporated nickel 4-point probe, ZT can be measured using the Harman Method. b) Nickel produces an ohmic contact to Bi 2 Te 3. c) Freshly deposited Bi 2 Te 3 nanowires show ZT = the highest ZT measured for nanowires of any composition. Figure 2. a-c) Bi 2 Te 3 nanowires prepared using using 10, 20, and 50 electrodeposition/stripping scans. d-e) Bi 2 Te 3 nanowires prepared by electrooxidation at V for d) 200s, e) 600s, and f) 800 s. The initial diameter of these nanowires was 130 nm + 15 nm. Figure 1. Synthesis of Bi 2 Te 3 nanowires on graphite step edges.

In 2001, we established an outreach program in connection with our NSF-funded research. This program brings local Irvine high school students (10 so far) to our labs to carry out concise research projects under the direct supervision of graduate students. These projects had a typical duration of mos, and encompass at least one summer. Often, these students present posters on their work at the Orange County Science Fair, and several of these students proceeded to the California State Science Fair where they have received some of the top awards. Two of these students have been recognized as Siemans-Westinghouse semifinalists. Our program is summarized by the table shown at right. During summer 2006, Dat Hoang (Trabuco High School, Mission Viejo) and Chasen Clark (UniHigh, Irvine) are participating in this program. Chasen Clark and Dat Hoang, Aug Outreach to Irvine High School Students Reginald M. Penner DMR

Swetha Kambhampati joined my research group in 2004 as a high school student at Irvine’s University High School, located near the UCI campus. Swetha spent two years with us, working under the supervision of graduate student Erik Menke who was supported by NSF DMR and who just received his Ph.D. in June, Their research project involved the development of a new method for synthesizing nanowires of a thermoelectric material, Bi 2 Te 3, summarized on this nugget. Swetha learned both how to synthesize Bi 2 Te 3 electrochemically using an electrodeposition method that involved scanning the electrode potential to first produce a bismuth-rich Bi 2 Te 3 deposit, and then electrochemically oxidizing the deposit to remove excess bismuth. She also learned how to image her nanowires using scanning electron microscopy (see picture at right). Using the technique Erik developed, but working independently from him, Swetha produced her own set of optimized synthesis parameters, based upon her own analyses of these nanowires by EDX (electron-excited, x-ray fluorescence elemental analysis). These data formed the basis for posters she prepared and presented. Her work culminated in the awards listed below: 1.Orange County Science & Engineering Fair, 2005, 2nd place. 2.California State Science Fair, 2005, 3rd place. 3.Siemans-Westinghouse 2005/06 semifinalist. 4.SINAM Outstanding Nanoscience Award - 1st place, presented by the Presented by the Center for Scalable and Integrated Manufacturing (SINAM) at UCLA. Swetha graduated from UniHigh this spring and she begins her freshman year at MIT in fall Swetha Kambhampati - Profile of a Penner Group High School Outreach Student Reginald M. Penner DMR