Intellectual Merit: The overarching research objective of this NSF proposal is to test the hypothesis that precise control of nanowire surface chemistry during growth can enable the rational engineering of semiconductor nanowire crystal structure. Successful validation of this hypothesis would improve the prospects for fabricating highly efficient nanowire-based photovoltaic and thermoelectric energy conversion devices. As opposed to the trial-and-error approaches that dominate the field, a custom-built in-situ infrared spectroscopic experimental platform will be utilized during these studies to fundamentally interrogate the chemistry that governs bottom-up semiconductor nanowire synthesis and structure. The combination of an ultrahigh vacuum growth environment and in-situ measurement is transformative in its ability to limit nanowire degradation and access a level of chemical detail not previously achievable. It will be possible to clearly distinguish which surface species or combinations of surface species, as well as what specific bonding structures, most strongly influence nanowire structure. Si nanowires serve as a technologically relevant model system and will be the focus of this work, but key findings will be broadly applicable to a range of important semiconductor nanomaterials. The specific objectives are three fold: (1) determine the role of surface-bound hydrogen on nanowire crystal orientation, (2) control nanowire crystal structure through the organic functionalization of nanowire sidewalls during growth, and (3) modulate nanowire structure as a function of axial position to create novel superstructures. Results from these objectives will collectively serve as the basis for semiconductor nanowire chemistry-structure and structure-property relationships that are essential for realizing devices with advanced performance. Preliminary data has demonstrated the importance of surface chemistry during nanowire growth and provides a strong foundation from which to begin this research effort.

Broader Impact: Breakthrough photovoltaic and thermoelectric technologies that could be widely deployed could transform our energy systems and reduce their carbon footprint. The insight gained during this work will greatly accelerate the design of nanoscale components for next generation devices, making a significant contribution to the United States declared goal of reducing carbon emissions over the coming years. Although this effort will be focused on energy conversion, advancements are expected to have broad applicability in a range of fields including photonics, electronics, quantum computation, and electrochemistry. In addition, students at all levels of STEM education, as well as the general public, will be prepared to intelligently discuss and navigate the emerging renewable energy landscape. The use of the Internet and social networking tools will directly integrate key outreach efforts into the daily activities of a broad, multidisciplinary audience. In particular, the Prof. Solar blog will be used to disseminate recent scientific and technological advances via short video vignettes produced in a format that leverages everyday experiences in an educational and entertaining manner. The PI is also involved in local outreach activities that focus on underrepresented minority students at the K-12 level. These efforts will be continued and broadened through speaking engagements and hands-on scientific demonstrations. In addition, Georgia Tech undergraduate students will have opportunities to participate in research internships.

Project Report

Nanoscale inorganic crystals find widespread application in nearly all areas of physical science. Semiconductor nanowires are a quintessential class of these materials and the vapor-liquid-solid (VLS) technique, where a liquid catalyst droplet collects semiconductor atoms from the vapor and directs the crystallization of each solid layer, is a workhorse synthesis method. Unfortunately, major synthetic challenges, stemming from an inadequate understanding of key mechanistic details and the use of chemistries originally developed for 2-D films, prevent robust programming of nanowire structure and restrict the accessible property space. The studies of VLS growth completed via this grant definitively show that surface chemistry plays a critical role in nanowire growth. Operando infrared spectroscopy was coupled with post-growth electron microscopy to connect specific chemical bonds with nanowire crystal structure, providing the fundamental chemical insight needed to rationally design growth precursors and improve synthetic design. Investigations of Si nanowire growth connected surface hydrogen and "kinks" for the first time (Shin and Filler, Nano Lett. 2012). These structural motifs, first reported more than four decades ago, form when nanowires change growth direction. Dr. Filler correlated hydrogen coverage with a transition from <111> to <112> oriented growth for Au-catalyzed Si nanowires, providing a previously absent chemical justification for this behavior. Separate experiments provided an explanation for the long-range structural coherence of nanowire kinking superstructures – nanowires containing multiple, well-defined kinks – observed only for Au-catalyzed Si nanowires (Shin et al., ACS Nano 2014). While surface hydrogen selects a family of growth directions, a twin plane running along the axial direction reduces the symmetry of the growth front and forces the nanowire to choose from two rather than many energetically equivalent options. This work establishes a general prescription for engineering kinking superstructures, and prompts a search for surface-active chemistries with which to direct growth direction as well as induce symmetry-breaking structures. The rational introduction of planar defects, specifically twin planes and stacking faults, in Si nanowires was also demonstrated (Shin et al., Nano Lett. 2013; Shin et al., ACS Nano 2013). Despite the pejorative connotation that accompanies the word "defect," these structural motifs present an exciting opportunity to reprogram atomic stacking sequence and, in doing so, tune the physical properties of many well-known semiconductors. Defects were occasionally seen for group IV nanowires prior to this research, but with a frequency far lower than their group III-V counterparts. It was shown that a reduction of growth temperature and increase in Si2H6 partial pressure can drive twin plane nucleation. These results provide strong evidence that III-V nanowires are not unique in their defect injection capability, only that the basic nanowire synthesis chemistry is "pre-tuned" to favor the process. The fabrication of prototype user-defined twinning and stacking fault superstructures with this new process sets the stage for modifying the atomic-level structure and function of Si, the world’s most widely used semiconductor.

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