Phase dependent visible to near-infrared photoluminescence of CuInS₂ nanocrystals

Abstract

Nanocrystals of CuInS 2 with the hexagonal wurtzite structure hold great potential for applications requiring efficient energy transport, such as photocatalysis, due to their anisotropic crystal structure. However, thus far their optical properties have proven difficult to study, as luminescence from wurtzite nanocrystals has only recently been observed. In this work, we report the colloidal synthesis of single crystalline, luminescent CuInS 2 nanocrystals with both the cubic and hexagonal structures. The crystalline phase, optical properties and mechanism of formation of nanocrystals are controlled by changing the reaction temperature. Photoluminescence is observed in the visible and near-infrared spectral regions, which results from the cubic and hexagonal nanocrystals respectively. Synthetic studies combined with XRD, TEM and EDS mapping provide evidence for the mechanisms behind phase selection. † Electronic supplementary information (ESI) available: Complete Rietveld refinement parameters; Scherrer line broadening crystallite size compared to TEM size; EDS data; quantum yields; Fityk peak fitting parameters; absorbance spectra of hexanes; TEM images from aliquot study; complete EDS maps for aliquot study, including dark field and elemental analysis data. See structure formed at these temperatures. 32 The QY was o0.8% for all samples (Fig. S5, ESI †).The visible peak, attributed to ZB CuInS 2 , was fit to a single Gaussian (Fig. S6, ESI †), which showed a red shift as reaction temperature increased. This is most likely due to an increase in the size of the NCs and the relaxation of quantum confinement conditions. This is consistent with the trends observed in absorbance spectra and reported in literature examples. [56][57][58] The broad, multimodal shape of the NIR PL peak, centred at B950 nm, and the large Stokes shift (0.25 eV) indicate that Fig. 3 (a) XRD of CuInS 2 NCs prepared at various temperatures. Pure ZB and WZ spectra are digitized from the work of Chang et al.; 36 (b) Proportion of WZ (blue), ZB (red) and hiCC Cu 2 S (green) phases present in each sample plotted as a function of temperature, as determined by Rietveld refinement of XRD; (c) EDS map of CuInS 2 NCs prepared at 155 1C showing the presence of Cu 2 S NCs; (d), (e) High resolution TEM (HRTEM) images of CuInS 2 NCs prepared at 215 1C (plates) and 115 1C (spheres) respectively showing lattice fringing.Fig. 4 (a) Absorbance spectra of CuInS 2 NC dispersions in hexanes prepared at different temperatures; (b) Tauc plot of the predominantly WZ CuInS 2 NCs prepared at 215 1C; (c) PL spectra normalized to the QY of CuInS 2 NC dispersions in hexanes prepared at different temperatures.

Folded Nanosheets: A New Mechanism for Nanodisk Formation

“…25 In 2 S 3 nanostructures have also been used as a template for the growth of CuInS 2 (CIS). 26,27 Among the various Cu-based multinary semiconductors, CIS has been established as one of the most promising candidates 28,29 owing to its direct band gap of ~ 1.5 eV, near ideal for harvesting visible light. 30 Rogach et al incorporated Cu + into In 2 S 3 nanoplates, resulting in CIS with tunable band gap that was used as an efficient counter electrode in dye-sensitized solar cells.…”

Shape-Controlled Synthesis of Copper Indium Sulfide Nanostructures: Flowers, Platelets and Spheres

“…This feature also accounts for the strong diffraction peak of the (002) plane of the wurtzite crystal pattern seen in the XRD pattern (Figure 2, red trace). The thickness of the CIS NPLs is therefore along the <100> direction, which is the a-axis of the wurtzite crystal structure; this is different to the orientation in the previously reported CIS NPLs [24,25]. The thickness is an important parameter for 2D nanostructures, as the quantum size confinement effect can be tuned via the thickness control in such materials.…”

“…CdSe NPLs with different numbers of layers have been extensively studied, and have showed strikingly narrow absorption and fluorescence spectra [5,7]. For the copper-based chalcogenides, some 2D nanostructures have also been reported, such as CIS hexagonal nanosheets synthesized via a cation exchange method or produced by so-called bottom-up synthesis methods [24,25]. These previously reported CIS nanosheets had wurtzite crystal structures, and the thickness of the NPLs extended along the <001> direction, which is the c-axis in the wurtzite structure.…”

CẤU TRÚC VÀ TÍNH CHẤT QUANG CỦA NANO TINH THỂ BÁN DẪN ZnxCd1-xS

“…Marray và các cộng sự khi chế tạo NC CdS, CdSe, CdTe nhận thấy rằng tại nhiệt độ cao, kích thước lớn của các NC và liên kết lỏng lẻo của ligan với bề mặt của NC có thể là nguyên nhân dẫn đến sự tạo thành các NC có cấu trúc Wz, trong khi đó cấu trúc Zb kém bền hơn về nhiệt động học thường nhận được khi chế tạo tại nhiệt độ thấp ( ≤ 240ºC) [9]. Leach và các cộng sự khi chế tạo hợp kim CuInS 2 cũng nhận thấy nhiệt độ chế tạo ảnh hưởng đến sự thay đổi cấu trúc tinh thể, khi nhiệt độ 115ºC các NC có cấu trúc tinh thể kiểu Zb, nhưng khi nhiệt độ tăng từ 115ºC đến 135ºC các NC có cấu trúc tinh thể kiểu Wz [6]. Nag và các cộng sự nghiên cứu vai trò của ligan lên đối với cấu trúc của NC CdSe, CdS nhận thấy oleic acid (OA) có vai trò ổn định pha cấu trúc Wz [7], trong khi đó trioctylphosphine (TOP) có vai trò ổn định pha cấu trúc Zb [9].…”