Size-Dependent Properties of Sonochemically Synthesized Three-Dimensional Arrays of Close-Packed Semiconducting AgBiS₂ Quantum Dots

Abstract

3D arrays of close-packed AgBiS 2 quantum dots (QDs) in thin film form were synthesized for the first time using novel, convenient sonochemical approach. Structural, optical, and photoelectrical properties of the synthesized material were investigated with an emphasis on their dependence on crystal size. The sonochemically synthesized AgBiS 2 colloidal crystals have an average QD radius of 4.2 nm, twice as small compared to the QD solid obtained without ultrasonic irradiation. The optical band gap energy of sonochemically synthesized AgBiS 2 QD thin films of 1.40 eV is strongly blue-shifted in comparison to that of the macrocrystal (0.90 eV) and that of nanostructured films synthesized by conventional chemical route (1.10 eV). Upon annealing, E g exhibits a red shift to 1.00 eV. Spectral dependence of stationary nonequilibrium conductivity of the 3D QD assemblies suggests that the thin films' photoconductivity is modulated by the intercrystalline barrier height decrease. E g of the films calculated on the basis of photoconduction spectral response in the low-absorption region is 1.18 eV. Thermal band gap energy of the films is 1.10 eV, whereas both the variable range hopping conduction and thermionic emission mechanisms are predominant in the overall intercrystalline charge carrier transport through 3D QD assemblies.

Sonochemically assisted colloidal route to CdSe quantum dot assemblies: an alternative way to further fine-tune the size-dependent properties

“…In the case of bulk cadmium selenide, the thermodynamically most stable modification at ambient temperature and pressure is the hexagonal (wurtzite) polymorph of this compound. The situation, however, that we encounter in the present case, is similar to those that we have reported before in the case of other compound semiconductor systems [59][60][61]. Namely, as a consequence of the ultrasmall dimensions of nanocrystals constituting the 3D array in the form of compact thin film or bulk precipitate, it is the cubic form that is actually being observed under ambient conditions.…”

“…We have further proceeded with careful analysis of the semiconductor absorption function of the form (28) to determine the band gap energy of the films deposited under various conditions, as well as of the thermally annealed films. We did this by a combined interpolation-extrapolation procedure, described in details in our previous publications [59][60][61]73]. It is necessary to implement such algorithm as the parabolic approximation for the dispersion relation is, strictly speaking, valid only in the neighborhood of the borders of the first Brillouin zone.…”

Matildite versus schapbachite: First-principles investigation of the origin of photoactivity inAgBiS2

“…However, more recent experiments report a reduced value of 1.3 eV even for smaller nanoparticles of 4.62 ± 0.97 nm size [1]. Other bulk E g measurements report a value 1.2 eV [5], which would limit the extend of quantum confinement effects; indeed some authors report E g values of 1.32 eV for ~16 nm diameter samples [7], and values of 1.11 eV for AgBiS 2 thin films [13], in accordance to optical measurement of 1.10 eV [14]. The above discussion does not limit to E g .…”

Applications of Ultrasound to the Synthesis of Nanostructured Materials

“…Various metal chalcogenides (e.g., CdS, [77,78] ZnS, PbS, [79] MoS 2 , [80,81] Bi 2 S 3 , [82] CdSe, [77] ZnSe, [83] PbSe, [84] Bi 2 Se 3 , [85] b-CuSe, [86] Cu 3 Se 2 , [86] Cu 7 Te 4 , [87] Cu 4 Te 3 , [87] GaSb, [88] AgBiS 2 [89] , etc) have been prepared by sonochemical synthesis. A typical synthesis of these materials involves the ultrasonic irradiation of an aqueous solution of a metal salt and a chalcogen source (e.g., thiourea for sulfur or selenourea for Se): in situ generated H 2 S or H 2 Se by sonication reacts with metal salts to produce metal chalcogenide nanoparticles.…”