Probing Electronics as a Function of Size and Surface of Colloidal Germanium Nanocrystals

Abstract

Inorganic semiconductor nanoparticles are of significant interest for applications that benefit from their size-dependent properties. The work presented here focuses on the characterization of solution-based microwave synthesized Ge nanocrystals (NCs).Three differently capped Ge NCs were investigated: oleylamine (OAM), dodecanethiol (DDT), and a functionalized N4,N4,N4',N4'-tetraphenylbiphenyl-4,4'-diamine (TPD) ligand, which is commonly used as hole-transporting units. The optical gaps followed the expected trend for quantum confinement; however, the absolute value depended upon the ligand. We found that the DDT-capped Ge NCs feature consistently larger bandgaps than OAM-capped Ge NCs of a similar size. Cyclic voltammetry (CV) was used to determine the valence band energy for OAM-capped Ge NCs, and the conduction band energy was estimated from the optical gap. By contrast, DDT-capped Ge NCs and the OAM/DDT-capped Ge NCs did not exhibit an oxidative signal in the cyclic voltammetry. This was attributed to the removal of surface defects of OAMcapped Ge NCs through stronger Ge-S surface bonds. TPD-capped Ge NCs were investigated and showed a shift to slightly higher oxidation potential compared with the free ligand and bandgap values in between that of the OAM-capped and DDT-capped Ge NCs. The higher oxidation potential is attributed to TPD orientation and the bandgap value reflects the lower number of Ge-S bonds on the surface due to ligand sterics.

Tuning the Photoluminescence of Germanium Nanocrystals through Surface Bound Functional Groups

“…Ligand‐induced changes in the optical properties of Ge NCs have been reported by Holmes et al, who showed a consistent 0.15 eV difference in the optical band gap of oleylamine and dodecanethiol capped NCs across a range of NC diameters (4.0, 5.4, and 7.6 nm) . They attributed these effects to changes in the atomic arrangements at the NC surface caused by different binding groups, which in turn modifies the band gap of the material.…”

“…Numerous approaches to combine synthesis and passivation of the Ge NC surface have been reported in the literature, including one‐pot methods, controlled synthesis, microwave heating, and even alloyed/doped Ge NCs . Several groups have reported the use of oleylamine as a capping ligand, while alkyl compounds, phosphines, and other functional groups have also been used .…”

“…Heat‐, UV‐, or catalyst‐initiated hydrogermylation reactions have produced alkyl‐terminated Ge NCs dispersible in nonpolar solvents as well as amine‐terminated Ge NCs dispersible in polar solvents. Reports in the literature have shown distinct differences in the UV–vis absorbance and photoluminescence (PL) spectra of Ge NCs terminated by different functional groups, indicating that the choice of ligand directly affects the optical properties of nanocrystals.…”

Structures, Oxidation, and Charge Transport of Phosphorus‐Doped Germanium Nanocrystals

“…[23][24][25][26][27][28] As another type of important elemental semiconductor NCs, Ge NCs hold a unique position because of a series of advantageous materials properties, including strong optical absorption and high mobilities of electronic carriers. [ 29 ] Although Ge NCs can now be controllably synthesized, [30][31][32] the doping of Ge NCs has been rarely explored. The delayed development of Ge NCs in doping may seriously limit the realization of the full potential of Ge NCs in a variety of applications.…”

Investigation of the photophysical properties of energy-relevant inorganic nanocrystals

“…where the precursor ratio controls the particle size. 46,[69][70][71][72][73] The polymerization of [Ge9] 4or other related Zintl ions, both with and without linking cations such as Ge 4+ or Pt 2+ , generates highly ordered, porous Ge nanocrystals. [74][75][76][77][78] Other preparations involve reduction of Ge-rich oxides, [79][80][81][82][83][84] heat-assisted reduction of the GeH2 Wittig adduct Ph3PCMe2·GeH2·BH3, [85][86] laser photolysis of Ge(CH3)4 or GeH4 gas, 87-89 photolysis of Ge wafer, 90 electroless deposition on preformed Ag nanocrystals, 91 Au-catalyzed vaporliquid-solid growth using GeH4 92 or diphenylgermane, 93 ultrasonic aerosol pyrolysis of tetrapropylgermane, 94 solution or solid phase reduction of NaGe, 95 plasma decomposition of GeCl4 [96][97][98] or GeH4, 99 sulfur-assisted thermal decomposition of triphenylgermanium chloride, 100 and heating a solution of an alkylgermane in various high-temperature organic solvents.…”