Colloidal Bi₂S₃Nanocrystals: Quantum Size Effects and Midgap States

Abstract

Among solution-processed nanocrystals containing environmentally benign elements, bismuth sulfi de (Bi 2 S 3 ) is a very promising n-type semiconductor for solar energy conversion. Despite the prompt success in the fabrication of optoelectronic devices deploying Bi 2 S 3 nanocrystals, the limited understanding of electronic properties represents a hurdle for further materials developments. Here, two key materials science issues for light-energy conversion are addressed: bandgap tunability via the quantum size effect, and photocarrier trapping. Nanocrystals are synthesized with controlled sizes varying from 3 to 30 nm. In this size range, bandgap tunability is found to be very small, a few tens of meV. First principles calculations show that a useful blueshift, in the range of hundreds of meV, is achieved in ultra-small nanocrystals, below 1.5 nm in size. Similar conclusions are envisaged for the class of pnictide chalcogenides with a ribbon-like structure [Pn 4 Ch 6 ] n (Pn = Bi, Sb; Ch = S, Se). Time-resolved differential transmission spectroscopy demonstrates that only photoexcited holes are quickly captured by intragap states. Photoexcitation dynamics are consistent with the scenario emerging in other metal-chalcogenide nanocrystals: traps are created in metal-rich nanocrystal surfaces by incomplete passivation by long fatty acid ligands. In large nanocrystals, a lower bound to surface trap density of one trap every sixteen Bi 2 S 3 units is found.

Space charge limited current mechanism in Bi2S3 nanowires

“…An overall trap density of the order of 10 20 cm À3 was assigned to the contribution of surface states caused by incomplete surface passivation. 35 From our data, the calculated trap concentration is of the order of 10 16 cm À3 . Thus, it can be concluded that the determined trap concentration at temperatures below 160 K could originate from the nanowire bulk.…”

Correlated electron–hole plasma in organometal perovskites

“…source (Quantronix Integra series), delivering 130-fs-long pulses in KHz repetition rate trains, 1 mJ energy per pulse 57 . Samples were kept in vacuum and the optical emission was dispersed by a spectrometer (Acton 2300 series with a 50 g mm À 1 , 600 nm blaze grating) coupled to a streak camera (Hamamatsu C5640).…”

Poly(ionic liquid)‐Mediated Morphogenesis of Bismuth Sulfide with a Tunable Band Gap and Enhanced Electrocatalytic Properties

“…However, for some applications such as photocatalytic water splitting, this narrow E g is not sufficiently large to drive the uphill reactions with theoretical energy requirements >1.23 eV plus the known overpotentials (Supporting Information, Figure S8). Our PILs‐controlled Bi 2 S 3 crystals show a striking increase of the E g (Figure ), presumably due to the quantum size effects arising from a reduction of the crystal size . An ultraviolet–visible (UV/Vis) spectrum (Figure ) reveals that the Bi 2 S 3 nanowires show an intrinsic semiconductor‐like absorption in the red region of the visible spectrum with an E g of 1.95 eV (Supporting Information, Figure S9).…”