59th Annual Report on Research 2014

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Reports: UNI1053401-UNI10: Probing Phonons in Low-Dimensional Thermoelectric Materials by Raman Spectroscopy

In this research, we used Raman spectroscopy to study laser induced oxidation, vibrational, and optical properties of stoichiometric and non-stoichiometric Bi₂Te₃ nanoplates (NPs). Bi-Te nanoplates with different thicknesses were grown by low pressure vapor transport method by our collaborators (Dr. Xuan Gao’s group) at Case Western Reserve University. In our studies, we used a Horiba Labram Raman microscope system and focused the laser to a spot with a diameter of ~1 μm. Other experimental techniques-atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDS), and Auger electron spectroscopy (AES)-were also used to characterize the sample properties. In relatively thick (>50 nm) stoichiometric Bi₂Te₃ NPs, we found that the crystalline structure is stable and sample surfaces do not show any damage under low laser power (much lower than 1 mW) irradiation. Raman spectra show four characteristic peaks from crystalline Bi₂Te₃ structures (as highlighted by the four solid vertical lines in Fig. 1(a)). As the laser power increases to an intermediate level (~1 mW), the NPs are oxidized and form bumps on the surfaces (see the optical image in Fig. 1(b), and the AFM image and profile analysis in Figs. 1(c) and (d), respectively) possibly due to expanded crystal lattice. The oxidation is revealed by the emergence of Bi₂O₃/TeO₂ Raman lines (highlighted by dashed vertical lines in Fig. 1(a)) and the increase of oxygen concentration in an EDS map (Fig. 1(h)) of the NP. Further increase of laser power not only causes oxidation (see Figs. 1(a) and (g) for Bi₂O₃/TeO₂ Raman lines, Fig. 1(b) for the optical image, and Fig. 1(h) for EDS map of oxygen concentration), but also burns holes on the sample surface. The AFM image and step-height analysis in Figs. 1(c), (e), and (f) show that the higher the laser power, the deeper the holes are. In NPs that are thinner than 20 nm and grown by the same method, we found that the Raman modes are different from those of stoichiometric Bi₂Te₃ crystalline structures and are consistent with those from Bi-rich Bi-Te materials. From the relative intensity between P2 and P3 modes (see Fig. 2(a)), we estimate that the Bi concentration is between 40-57% in our thin NPs. We confirmed the stoichiometries of two thin NPs by AES, which showed excellent agreement with those estimated using Raman method. We found that the laser induced oxidation is not prominent in these non-stoichiometric thin NPs. We also found that the optical absorption of the thin NPs strongly depends on their stoichiometry. NPs with the same thickness but different stoichiometries show very different color contrast compared to the SiO₂ substrate (see Figs. 2(b) and (c)). We estimated the optical absorption coefficient of thin Bi-Te NPs with different stoichiometries by comparing the intensities of the Si Raman lines when the laser is directly incident on the substrate and after it penetrates the NPs. Figure 2(d) shows the optical absorption coefficient as a function of the relative intensity between P2 and P3 and as a function of Bi concentration. Our results show that thin Bi-Te NPs grown by the low pressure vapor transport technique show various stoichiometries that differ from Bi₂Te₃, and that the optical properties of these NPs is strongly influenced by their stoichiometry. Therefore, controlling the stoichiometry in the Bi-Te NP growth is important for their thermoelectric, electronic, and optical device applications. Details of these findings are to be published in Nano Research. A new closed cycle optical microscopy cryostat was installed in the PI’s lab in June 2014. This equipment, together with the Raman microscope system, has enabled variable temperature optical microscopy studies of diverse materials. Figure 3 shows preliminary data of temperature dependent Raman spectra from a Bi₂Te₂S single crystal (samples were provided by Dr. Yong Chen’s group at Purdue University). Students will analyze the data under the PI’s supervision in the coming academic year. This PRF grant has allowed the PI to explore a new research area that she has initiated since she started her career as an assistant professor at the University of Northern Iowa (UNI). The research has stimulated vigorous internal (within the PI’s department) and external collaborations that connect UNI, a predominantly undergraduate institution, with Case Western Reserve University and Purdue University, Ph.D.-granting research institutions. Five papers were published during this PRF grant support period (see Publication list) and one paper was recently accepted for publication in Nano Research. The research projects enabled by this grant open up new opportunities for our undergraduate students to improve their preparation for graduate school and the STEM workforce. Four physics major undergraduates (Casie Means-Shively, Courtney Keiser, Chao Ji, and Zhipeng Ye) were supported by the PRF grant and participated in the research activities. A new student, Heidi Anderson, started her research in August 2014 and is also supported by the PRF grant. Courtney Keiser and Zhipeng Ye’s research in the PI’s group was supported by PRF since fall 2013. They are coauthors on two papers published during this grant active year. Courtney Keiser attended the 2014 American Physical Society March Meeting held in Denver Colorado and presented the research on Bi₂Te₃ nanoplates at the meeting (see Fig. 4(a)). Chao Ji and Casie Means-Shively joined the PI’s group in summer 2014 and participated in the research of temperature dependent Raman studies of phonon properties of doped Bi₂Te₃ crystals (see Fig. 4(b)). This research experience offered by the PRF grant has stimulated our students to pursue advanced education in graduate school to further their education.