Mentoring relationships can be tremendously valuable for both the mentors and their protégés; however, these partnerships may be difficult to establish without a designated mode of communication. We will discuss our implementation of such a venue within the Department of Materials Science and Engineering at Stanford University to both address undergraduate concerns and train graduate students to be future mentors.

We designed a mentorship program with graduate student mentors and undergraduate student protégés to facilitate dialogue and spread ideas between the various communities in the department. While these groups of people see each other daily, we found that interaction was rare outside of a teaching assistant - student type of exchange. Furthermore, undergraduate students were seeking additional resources to help realize their post-graduate goals. Graduate students in turn had recently navigated similar situations successfully and were willing to engage. Our implementation of the program received wide departmental support, with 19 graduate students and 15 undergraduate students (about three quarters of the undergraduate materials science student body) requesting to participate. We formed three types of test mentorship groups:Type I. 7 traditional groups of one mentor and one mentee,Type II. 4 groups of two mentors and one mentee,Type III. 2 groups of two mentors and two mentees.

We paired groups based on common interests and goals, along with preference for particular mentoring styles.

This 8-month program was divided into an introduction stage (February), a meeting period (March through September), and a final recognition event (October). During the introduction stage, we facilitated a welcome event that included goal-setting exercises, icebreakers and conflict resolution strategies. During the next 6-month span we asked groups to meet at least twice for unstructured events. We created an official Stanford course website as a channel to communicate with participants. Finally, we hosted a dinner to recognize achievements and gather feedback for this pilot program.

We will discuss execution, reception and assessment of this inaugural program. The hosted events were positively received, and nearly all groups requested additional structured events. We further found that in Type II and III groups, the mentor-mentor relationship was an important factor influencing group dynamics and sustained interest in the program. While assessment is often challenging in volunteer-related activities, we will provide suggestions to gauge progress and share techniques for successful implementation at other universities.

This work was funded by the MRS Special Projects Initiative and the Stanford Department of Materials Science and Engineering.

High permeability whilst keeping a reasonable selectivity is the most important challenge in developing membrane systems for gas separation. Valuable performances are usually obtained with polymeric membranes for which the gas transport is controlled by the gas-diffusivity in glassy polymers and by gas-solubility in rubbery polymers. During the last decade, important advances in this field are related to the molecular control of the gas separation properties. The combination/replacement of classical glassy polymers with metal-organic crystalline frameworks (MOFs, ZIFs, zeolites…) providing reasonable permeability through porous free volume network and high selectivity due to so-called “selectivity centers” specifically interacting with the gas molecules.

Despite the impressive progress, important difficulties are observed to get dense mechanically stable thin layer MOFs on various supports. Taking advantage of high permeabilities observed with the rubbery polymers and to their flexible casting properties, there should be very interesting to build rubbery organic frameworks-ROFs, as alternative for gas membrane separation systems. Here we use low macromolecular constituents and dialdehyde core connectors in order to constitutionally generate rubbery organic. Differently to rubbery polymeric membranes the ROFs performances depend univocally of diffusional behaviors of gas molecules through the network. For all gases, a precise molecular composition of linear and star-type macromonomers generates an optimal free volume for a maximal diffusion through the matrix. These results should initiate new interdisciplinary discussions about highly competitive systems for gas separation, constitutionally controlled at the molecular scale.

1. M. Barboiu, , Encyclopedia of Membrane science and Technology, Review, 2013, Wiley.

2. G. Nasr, A. Gilles, T. Macron, C. Charmette, J. Sanchez, M. Barboiu, Israel J. Chem. 2013, 53, 97-101.

3. G. Nasr, T. Macron, A.Gilles, C. Charmette, J. Sanchez, M.Barboiu, Chem. Commun., 2012, 49 11546.

4. G. Nasr,T. Macron, A. Gilles , Z. Mouline, M. Barboiu, Chem. Commun.,2012, 48, 6827-6829.

5. G. Nasr, T. Macron, A. Gilles, E. Petit, M. Barboiu, Chem. Commun., 2012, 48, 7398-7400.

Advanced nanofabrication techniques have enabled rapid advancements in creating plasmonic nanoparticle arrays with complex optical resonance properties that are determined by their intrinsic geometry and extrinsic environment. These structures have been adopted for a wide range of applications that rely on either near-field enhancement or far-field diffractive coupling, including waveguides, sensors, solar cells, and metamaterials. The most common approach to fabricate nanoparticle arrays employs conventional top-down lithographic patterning followed by lift-off of an evaporated plasmonic metal. Although this method provides excellent control of nanoparticle size and placement, the lift-off process creates cylindrical particles with asymmetric tilted sidewalls, which can broaden the plasmonic peak width compared to an ideal isotropic spherical particle array.

An alternative strategy has used localized laser induced heating of a planar evaporated Au film to create 2D spherical plasmonic particle arrays. However, the minimum spacing between adjacent particles has been limited to the micron scale, which prevents fabrication of arrays with strong interparticle coupling. This presentation will describe a new nanofabrication method that employs Au-enhanced oxidation to synthesize 2D spherical plasmonic nanoparticle arrays with well-controlled particle placement, diameter, and spacing down to the nanometer scale.

The process begins by defining an array of cylindrical amorphous-Si/Au nanoparticles on a fused silica substrate using electron-beam lithography and evaporation. The amorphous-Si/Au particle array is then converted into a Au/SiO2 core-shell nanoparticle array by thermally oxidizing the entire structure in dry O2. During the thermal treatment, the Au core is transformed from a cylinder into a sphere to reduce the interfacial energy between the Au core and the SiO2 shell. In this process, the final spherical Au nanoparticle diameter and interparticle spacing is determined entirely by the starting lithographic pattern and the evaporated Au volume.

To evaluate the optical properties of these structures, the angular response of a 2D periodic spherical nanoparticle array with 135 nm diameter Au particles and 360 nm spacing was compared to a conventional evaporated Au cylindrical nanoparticle array with the same geometry. The spherical nanoparticle array had a narrower resonance peak width and coincident transmission spectra for transverse electric (TE) and transverse magnetic (TM) polarizations at normal incidence.

Additionally, the isotropic particle geometry resulted in congruent reflectance in both the TE and TM modes at oblique angles of incidence. This approach is also being applied to fabricate 2D aperiodic nanoparticle arrays with Ammann-Beenker and Penrose tilings, which lack transitional symmetry and thus excite multiple plasmonic/photonic hybrid modes for a broader resonance response.

More than 50% of total input energy is wasted as heat in various industrial processes. If we could harness a small fraction of the waste heat while satisfying the economic demands of cost versus performance, then thermoelectric (TE) power generation could bring substantial positive impacts. To meet these demands single-crystal semiconductor nanowire networks have been investigated as a method to achieve advanced TE devices because of their predicted large reduction in thermal conductivity and increase in power factor.

To further our goal of developing practical and economical TE devices, we designed and developed a material platform that combines a semiconductor nanowire network and a semiconductor thin film integrated directly on a mechanically flexible metallic substrate. We assessed the potential of this platform by using indium phosphide (InP) nanowire networks and a doped poly-silicon (poly-Si) thin film combined on copper sheets. InP nanowires were grown by metal organic chemical vapor deposition (MOCVD). In the nanowire network, InP nanowires were grown in three-dimensional networks in which electrical charges and heat travel under the influence of their characteristic scattering mechanisms over a distance much longer than the mean length of the constituent nanowires.

Subsequently, plasma-assisted CVD was utilized to form a poly-Si thin film to prevent electrical shorting when an ohmic copper top contact was made. An additional facet to this design is the utilization of multiple materials to address the various temperature ranges at which each material is most efficient at heat to energy conversion. The utilization of multiple materials could enable the enhancement of total power generation for a given temperature gradient. We will investigate the use of poly-Si thin films combined with InP nanowires to enhance TE properties. TE parameters, power production, and challenges of a large area nanowire device on a flexible metallic substrate will be presented. We will discuss our design and testing of a new large area, scalable, mechanically flexible TE devices.

Rapid thermal processing is widely used manufacturing process in the photovoltaic (PV) industry. However, the processing parameters have evolved empirically over time, mainly due to lack of understanding about the actual phase formation mechanisms during the processing which occurs in short time scales of few seconds to minutes.

For example, the Ag-Si contact formation begins with printing a mixture of an Ag powder, glass frit (mixture of metal oxide such as PbO, BO, ZnO and BiO) and an organic binder over the antireflection coating which is subsequently fired up to about 800°C. It is known that the frit allows the paste to react with and burn through the anti-reflective coating such that the metal can react with underlying c-Si during firing. However, the precise phase transformations between Ag, Si, SiNx, and frit constituents, which happens within few seconds (typically <10 s) during RTP, giving rise to optimal Ag-Si contacts are not well understood.

While there are several proposed mechanisms for Ag-Si cell contact formation during rapid thermal processing, there is no in-situ characterization in the actual processing conditions. We have established a rapid thermal processing/X-ray diffraction/fluorescence (RTP/XRD-XRF) facility, where we are able to monitor and characterize the Ag-Si cell contact formation with a time resolution of a fraction of a second. The facility utilizes the intense synchrotron X-ray source to gather structural and chemical information while material is being processed. We utilize a large fast area detector with few ms time resolution to gather a large solid angle diffracted beams, while an energy dispersive vortex detector for in-situ chemical analysis.

For long-term, economically competitive, sustainable solar energy to become a reality, absorber materials for photovoltaics should be selected for their earth abundance and low extraction costs. Iron pyrite, FeS2, has been proposed as an excellent candidate for these reasons, in addition to its reasonable band gap and large absorption coefficient. [1,2] Even though its photovoltaic properties were explored as early as the 1980’s and a recent resurgence in pyrite research has produced a number of publications, experimental studies have produced only poor results with regards to the photovoltaic properties. Many have attempted to explain the problems plaguing the material, but no experimental data has shown improved performance above the record 3% efficiency. [3]Recently, a class of materials has been proposed as an alternative to pyrite; Fe2MS4 (M = Si, Ge). Calculations have been used to predict nearly ideal photovoltaic properties (a band gap of 1.40 - 1.55 eV and a large absorption coefficient of 10^5 cm-1). [4] One hypothesis for the poor performance of pyrite is the possible decomposition of the desired phase to other iron-sulfur phases with very small band gaps. According to theory, there are no binary phases in the Fe2MS4 system that are calculated to be more stable than the ternary phase, suggesting it may be a significantly better candidate for solar cells because it may be more thermodynamically stable than the binary Fe-S pyrite phase. Historically, the bulk material has been studied for its interesting magnetic transitions as a function of temperature, but experimental reports of photovoltaic properties are limited. Herein, we report the synthesis of colloidal Fe2GeS4 nanocrystals for use in photovoltaic devices. The as-synthesized nanocrystals are phase-pure and form plate-like structures that show broad absorption in the visible region. While the nanocrystals exhibit poor stability under ambient conditions, methods of surface passivation for improved stability and enhanced photovoltaic properties will be discussed. These results pave the way towards a new earth-abundant material for use in low-cost photovoltaics.

[1] Wadia, C.; Alivisatos, A. P.; Kammen, D. M. Environ Sci Technol 2009, 43, 2072.

[2] Ennaoui, A.; Tributsch, H. Sol Cells 1984, 13, 197.

[3] Ennaoui, A.; Fiechter, S.; Pettenkofer, C.; Alonsovante, N.; Buker, K.; Bronold, M.; Hopfner, C.; Tributsch, H. Sol Energ Mat Sol C 1993, 29, 289.

[4] Yu, L.; Lany, S.; Kykyneshi, R.; Jieratum, V.; Ravichandran, R.; Pelatt, B.; Altschul, E.; Platt, H. A. S.; Wager, J. F.; Keszler, D. A.; Zunger, A. Adv. Energy Mater. 2011, 1, 748.

Charged or multiply-excited colloidal quantum dots (QDs) have typically low emission because of fast nonradiative Auger process and this impedes the development of QD based lasers, light emitting diodes, and photovoltaics. However, in the last few years, strongly reduced Auger process has been observed in ultra-thick-shell CdSe/CdS nanocrystals and dot-in-rod structures with large particle sizes (~20 nm) where the electron confinement was minimal. This has been qualitatively attributed to various mechanisms, surface effects or interfacial alloying, but it remained very unclear whether Auger suppression could be achieved in small colloidal QDs (~5 nm) where the quantum confinement effect is preserved. We therefore investigated the photoluminescence of charged colloidal dots with varying core/shell structures in order to design and tune the wavefunctions. With two electrons and one hole, the negative trion is arguably the simplest system to study the Auger process in colloidal QDs and, using confocal microscopy and controlled charging, we could investigate single dots in a defined -1 charge state at room temperature. In summary, type I CdSe/ZnS exhibit very short-lived trions, while with the weakly type II CdSe/CdS, the trion lifetime lengthens slowly with increasing shell thickness in accord with other results. However, the first study of the strongly type-II CdTe/CdSe QDs revealed that a long trion lifetime is obtained for very small particles and, uncharacteristically, that the trion lifetime is maximum (~4.5 ns) for an intermediate shell thickness. This leads to the smallest particles (~4.5 nm) with the brightest negative trion to date. At the single dot level, the negative trion exhibit non-blinking behavior and can reach almost the same brightness as the single exciton.

The unprecedented Auger suppression in such small dots is a significant departure from prior results with much larger nanoparticles. This leads us to a new proposal, whereas the optimum shell thickness in the type II structure is when the electron energy is exactly at the bottom of the conduction band of the core. Therefore, the Bloch functions for the electron and hole would have different central symmetries, leading to the vanishment of the transition matrix in the Auger process. We propose that this is also the main mechanism for the Auger suppression in the weakly type II CdSe/CdS, those needing a much larger shell to achieve the same effect. This study is a significant improvement in the understanding of multiexciton recombination in colloidal quantum dots, and it may impact the design of widely tunable small bright charged QDs and help develop efficient light-emitting devices.

The Center for Functional Nanoscale Materials (CFNM), an NSF Center for Research Excellence in Science and Technology, at Clark Atlanta University (CAU) is an ACS (American Chemical Society) Project SEED site. The ACS project SEED Program is recognized nationally as providing hands-on research opportunities to disadvantaged high school students, who have historically lacked exposure to scientific careers. CAU is a minority-serving institution and has an excellent working relationship with Atlanta area school systems that serve large numbers of minority students. Students entering their junior or senior year in high school are selected for the Program based on their academic performance, an essay and letters of reference from their teachers. These high school students then become part of CFNM’s eight week summer Nanoscholars Program.

The program included a weekly Nanoscience Survey sessions during which the Nanoscholars learned about scientific problem solving, nanotechnology, nanomaterials, and scientific reporting. The weekly Journal Club, facilitated by the professors and graduate students, involved discussions of articles on such topics as material science, nanotechnology and ethics in science. Each Summer Nanoscholar participated in the research program of his/her advisor and prepared and presented a scientific paper to the CFNM Community (participants, students, professors and mentors) at the end of the Program.

We have completed three summers as an ACS Project SEED site. So far we have had one SEED scholar submit a major manuscript, two were invited to present at ACS National Meetings and one was awarded an eight (8) year Gates-Millennium scholarship. The evaluation of the Project strongly suggests that our approach is effective for opening doors for the economically disadvantaged students and tapping the best and the brightest for careers in the sciences and engineering.

The evaluation component was organized around the following three dimensions: (1) Program Implementation (Effectiveness and efficiency of service delivery by the program); (2) Student Data (Academic and professional competencies, including research publications and presentations, lab performance, demonstrated leadership skills and satisfaction); and (3) Performance Feedback (Evaluations of all components to address the question, "How well is the CFNM Project SEED achieving its goals?"). In the words of one of our young scholars “I realized that research is a continuous learning process. You can never know everything. Even a professor has credentials but they’re still continuing to learn.”