AVS2015 Session EM+AS+EN+NS-FrM: Nanoparticles for Electronics and Photonics

Colloidal quantum dot (QD) solids are the subject of active research with applications emerging in light-emitting diodes, field-effect transistors, and solar cells. In this talk, I describe the use of atomic layer deposition (ALD) infilling to engineer the surfaces and interfaces of PbSe QD films in order to produce high-performance QD field-effect transistors (FETs) that completely lack bias-stress effect (i.e., drain current transients caused by charge trapping near the dielectric/channel interface). This ALD “matrix engineering” approach includes steps designed to manage ligand concentrations, passivate surface states, and arrest ionic motion within the films, resulting in the first high-mobility (~14 cm V^-1 s^-1), environmentally stable, and transient-free PbX QD transistors. Two bias-stress mechanisms in QD FETs are identified and discussed. The implications of these mechanisms for the operation of QD solar cells is highlighted.

In this work, we report the synthesis, characterization and manufacturing of Cu, Pt and Pd doped SnO₂ pellets with ultra high sensitivities for CO atmospheres. To the best of our knowledge, we have accounted for the first time the ultra high CO sensitivities for Cu doped than Pt and Pd doped SnO₂ pellets. In order to obtain high sensitivities, we have employed novel methods, which are the mixture of chemical and physical synthesis methods. Non-spherical SnO₂ structures were prepared via two chemical synthesis routes using Urea (R1) and ammonia (R2) as precipitation agents. The resultant SnO₂ powders were doped with transition metal, Cu, and noble metals like Pt and Pd via two doping methods D1 and D2. In D1, the powders were bulk doped and then ball milled, whereas in D2, the powders were ball milled and then surface doped. All the powders obtained were later pressed using manual pressing machine to manufacture the SnO₂ pellets. The effect of synthesis routes, doping methods and dopants, on the structural, morphological and also on CO sensing were studied by different characterization techniques and reported with their detailed explanations. Interestingly, the Cu-SnO₂ pellets manufactured from the powders obtained by method D1R1, showed highest sensitivity around 1783 due to various reasons like uniform and small particle size, necks formation, inter-particle conductance and high oxygen adsorption due to stacking faults. All the reasons mentioned above were demonstrated by comparing the established sensor theory with our different experimental results obtained using XRD, Raman, SEM, HRTEM and sensitivity analysis.

Controlled lateral ordering of self-assembled semiconductor quantum dots (QDs) is desirable for a wide range of solid-state applications, including solar cells, lasers, and telecom devices. To date, lateral alignment of QDs has been demonstrated for multilayers of QDs.^[1]

In these cases, the first layer of QDs is isotropically distributed; subsequently, during the growth of QD stacks, the accumulation of anisotropic strain often results in lateral QD alignment. However, a significant remaining question concerns the direct influence of spontaneous surface patterning on the selective nucleation of QDs.

In this work, we use a combined experimental-computational approach to directly examine correlations between buffer surface morphology and QD nucleation. For this purpose, we exploit a surface instability induced by the anisotropy of the surface diffusion constant of ad-atoms (the Ehrlich-Schwöbel effect^[2]) which leads to the formation of elongated ripples, often termed “mounds”. For epitaxial growth of InAs QDs on GaAs, Ye et al. reported a preference for in-plane QD alignment along the mound lengths^[1]. Here, our one-dimensional phase-field model reveals a preference for QD nucleation in regions of positive curvature,^[3] such as on the sides of the mounds and/or in the “valleys” between the mounds. In our experiments, we explore the formation of InAs QDs on AlGaAs mounds using various substrate temperatures and indium exposure times.

We explore the use of fixed geometry indium evaporation as an approach to restrict QD nucleation to one side of the AlGaAs mounds, resulting in the formation of 1D QD chains.^[4] Specifically, for substrate temperature of 580°C, a high density of AlGaAs mounds is observed along [0-11]. For 3 monolayer (ML) of InAs deposition, we achieved selective positioning of QDs, with an average diameter of 16nm, on one side of the mounds.

We will discuss the influence of the As species (As2 vs As4) and growth interrupts on the size, density, and spatial arrangement of QDs. We will also present a detailed analysis of the surface instabilities that induce ripple formation, and the As adsorption kinetics, which lead to the anisotropic nucleation.

References

[1] W. Ye, S. Hanson, M. Reason, X. Weng, and R. S. Goldman. (2005). J. Vac. Sci. Technol. B 23, 1736-1740.

[2] Schwöbel, R. L., & Shipsey, E. J. (1966). J. App. Phys., 37(10), 3682–3686.

[3] Seol, D. J., Hu, S. Y., Liu, Z. K., Chen, L. Q., Kim, S. G., & Oh, K. H. (2005) J. Appl. Phys., 98(4), 044910.

[4] Arciprete, F., Placidi, E., Magri, R., Del Gaudio, D., Patella, F. (2013) J. Mat. Res.28(23), 3201–3209

• Covalent bonded NPs, in particular carbon;
• Metallic NPs: Cu¹, Fe, Mg, Mo, Co, Al, Ag, Nb, Ti, Pd;
• Semiconductor NPs, in particular Ge;
• Bimetallic NPs: MgNi, MoCu, MgTi with several compositions;
• Ternary alloy NPs, e.g. GeSbTe with several compositions and with amorphous and crystallinity control.

The particles can be deposited on most surfaces provided they have good vacuum compatibility.

The applications range from novel:

• Wetting phenomena; Cu NPs covered surfaces giving rose petal effect²;
• Bimetallic Mo-Cu NPs which are bulk immiscible but in NPs fully miscible³;
• Bimetallic NPs for hydrogen storage⁴: MgNi, MgTi, MgCu;
• Magnetic NPs: Fe-Fe₃O₄ core-shell particles for medical applications.

1. Brink, G. H. ten, Krishnan, G., Kooi, B. J. & Palasantzas, G. Copper nanoparticle formation in a reducing gas environment. J. Appl. Phys. 116, 104302 (2014).

3. G. Krishnan, M.A. Verheijen, G.H. ten Brink, G. Palasantzas, B.J. Kooi, Tuning structural motifs and alloying of bulk immiscible Mo-Cu bimetallic nanoparticles by gas-phase synthesis, Nanoscale 5, 5375-5383 (2013).

4. Krishnan, G. et al. Synthesis and exceptional thermal stability of Mg-based bimetallic nanoparticles during hydrogenation. Nanoscale 6, 11963-11970 (2014).

We show the results of using a new surfactant, ammonium laurate (AL), to suspend and deposit carbon nanostructures. In a recent publication¹, we show that multi-walled carbon nanotubes (MWCNTs) can be suspended in AL with much better shelf stability as compared with the common surfactant sodium dodecyl sulfate (SDS). AL differs from SDS only by the choice of ionic species, but the deposition process with AL is more reliable and cleaner than with SDS. We use a process of producing a charged self-assembled monolayer on the substrate and then exposing the substrates to the aqueous surfactant solution of MWCNTs to achieve directed deposition of clean individual MWCNTs, which can then be used for fabrication of individual nanotube devices. In addition to these results, we show results for single-walled carbon nanotubes (SWCNTs) in AL, which show that nanotubes deposited from AL have lower electrical contact resistance as compared to those deposited from SDS. Photoluminescence results also show that SWCNTs with specific chirality are preferentially suspended in AL, which may present a separation and purification pathway. We will also present extensions of the work to single and few layer graphene sheets, where AL can be used to make clean depositions from aqueous solution onto sensitive substrates.

(1) Nilsson, H. M.; Meany, B.; Ticey, J.; Sun, C.-F.; Wang, Y.; Cumings, J. Ammonium Laurate Surfactant for Cleaner Deposition of Carbon Nanotubes. Langmuir 2015, 31, 6948-6955.

An innovative and scalable synthesis approach to the formation of stoichiometric Cu₂ZnSnS₄ (CZTS) nanocrystals has been developed using aerosol spray pyrolysis. This quaternary phase material is a potential replacement for currently commercialized semiconductors such as CdTe and CIGS that are used in photovoltaic devices. However, sustainability and environmental issues threaten long-term viability of these materials. Based upon earth abundant constituents and low chemical toxicity, CZTS, with a reported bandgap of ~1.5 eV^[1], appears to be a superior alternative to these other materials. Additional research and development is necessary to increase the efficiency of CZTS-based cells from the current record (12.6% by Wang et al.^[2]) to the >18% necessary to be considered commercially viable. Our work demonstrates the controllable, cost-effective, and reproducible synthesis of high-quality CZTS nanoparticles and films. A modified spray pyrolysis method involving decomposition of copper, zinc, and tin diethyldithiocarbamate precursors allows uniform incorporation of dopants (such as sodium) that are known to increase crystal grain growth during nanoparticle sintering^[3]. Once formed, the nanoparticles are deposited onto a substrate from a methanol dispersion using an “ink-spray” process with an argon-driven airbrush. To form an efficient absorber layer in a photovoltaic device, the coating is then annealed in a sulfur-vapor atmosphere resulting in a thin film with uniformly large crystal grain morphology throughout the film thickness (~1-2 µm). The deposited films are characterized with respect to crystalline phase, stoichiometry, and overall film quality. Further preliminary results regarding the formation of Cu₂ZnSn_(1-x)(IV)_xS₄ by means of this processing approach will be reported.

^[1] H. Wang. “Progress in Thin Film Solar Cells Based on Cu₂ZnSnS₄,” International Journal of Photoenergy 2011 (2011).

^[2] Wang, Wei, Mark T. Winkler, et al. “Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency.” Advanced Energy Materials 4, no. 7 (2014).

^[3] Johnson, M., S. V. Baryshev, et al. “Alkali-Metal-Enhanced Grain Growth in Cu2ZnSnS4 Thin Films.” Energy & Environmental Science 7, no. 6 (2014): 1931–38.