Plasma Synthesis of Highly Monodisperse Ge Nanocrystals and Self-Assembly of Dense Nanocrystal Layers
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Plasma Synthesis of Highly Monodisperse Ge Nanocrystals and Self-Assembly of Dense Nanocrystal Layers Ryan Gresback, Zak Holman, and Uwe Kortshagen Mechanical Engineering, University of Minnesota, 111 Church Street SE, Minneapolis, MN, 55455
ABSTRACT Germanium nanocrystals have been synthesized using a low-pressure, nonthermal plasma approach. The nanocrystal size can be adjusted between 4-20 nm by varying the plasma parameters, and the size distribution is relatively narrow with standard deviations of 10-20% of the average crystal size. Stable colloidal solutions of the germanium crystals have been prepared by grafting organic alkene ligands onto the nanocrystal surfaces. When drop-cast from solution onto TEM grids, the nanocrystals form densely packed films. INTRODUCTION Germaninum nanocrystals (Ge-NCs) may be interesting candidates for active layers in optical and electronic devices, such as quantum-dot-based solar cells. Theoretical1 and experimental2 studies suggest that the band gap of Ge-NCs can be altered from the bulk Ge band gap of ~0.7 eV to more than 2 eV by shrinking the nanocrystal size to ~2-3 nm in diameter. Thus, the band gap of Ge-NCs may be tuned across the range of energies relevant to photovoltaic devices. Moreover, unlike many other quantum dot materials that are currently enjoying popularity, Ge is non-toxic, environmentally benign, stable, and abundant. A wide range of synthesis approaches has been reported for Ge-NCs. Synthesis in the liquid phase3-8 often allows for good control of the particle size distribution and suppression of agglomeration. However, liquid synthesis routes are frequently time consuming and have low material yields. Ge-NCs can also be precipitated in the solid phase.9-11 While such methods yield Ge-NCs embedded in a matrix material that, in many cases, passivates the NC surfaces very well, they also limit the range of surface treatments schemes and the spectrum of routes that may be used to incorporate NCs into devices. Gas-phase synthesis12 produces free-standing Ge-NCs, whose surfaces are easily accessible for functionalization. Unfortunately, gas-phase processes are seriously affected by uncontrollable particle agglomeration, which may annihilate the desired NC properties. In the past, we showed that nonthermal plasmas may be used to synthesize sizecontrolled silicon NCs with large material yields, while simultaneously largely eliminating the problem of particle agglomeration.13 In this approach, a precursor gas at low pressure is dissociated through impact of energetic plasma electrons, leading to rapid chemical clustering of the produced radicals and subsequent NC formation. We pointed out that, based on the large mobility of plasma electrons compared to that of the ions,
NCs in a plasma acquire a unipolar negative charge, which either strongly retards or completely eliminates particle agglomeration. Moreover, we showed that strongly exothermic reactions at the NC surfaces, such as chemical reactions and electron-ion recombination, may lead to NC t
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