Type I vs. quasi-type II modulation in CdSe@CdS tetrapods: ramifications for noble metal tipping

Abstract

We report on noble metal tipping of heterostructured nanocrystals (NCs) of CdSe@CdS tetrapods (TPs) as a chemical reaction to manifest energetic differences between type I and quasi-type II heterojunctions. The energetics between zincblende (ZB) CdSe seed and wurtzite CdS TP arms has previously been probed primarily using ultrafast spectroscopic methods. However, to interrogate the energetics of CdSe@CdS TPs from both larger and very small ZB CdSe seeds (i.e., effective diameters (D_eff) 6.2 to 1.8 nm) we utilize the photodeposition of gold nanoparticles as a facile and selective chemical reaction to correlate reaction rates with tetrapod energetics. We observe a 20-fold enhancement in the rate of selective photodeposition of a single AuNP tip onto this series of TPs, which were attributed to straddling of type I vs. quasi-type II energetics as a direct consequence of varying ZB CdSe seed size. Synthetic access to type I and quasi-type II TPs enabled the synthesis of singly tipped AuNP-CdSe@CdS TPs in gram quantities, which is the largest scale synthesis of metal–semiconductor Janus nanoparticles to date

Hole Scavenging and Electron–Hole Pair Photoproduction Rate: Two Mandatory Key Factors to Control Single-Tip Au–CdSe/CdS Nanoheterodimers

“…The results comparisons are based on the deposition sites, such as single-tip (ST), double-tip (DT), and multisite (MS) distributions. The most effective methods now tend to lower the deposition temperature, ,,,,, shorten the reaction time, , and use long alkyl amine ligands; ,, laser or lamp-assisted photodeposition may also help in obtaining more selective ST deposition. , In some cases, the hole scavenger was stated as such and identified as a short-chain alcohol (methanol, ethanol, 2-propanol, ethylene glycol), ,− but most of the time it is barely mentioned, and TEM images show quantum dots and DRs corrosion taking place concomitantly with the gold deposition.…”

“…The reaction time has been mentioned to play a significant role in controlling the deposition. However, there are also results at variance: (i) Pavlopoulos et al showed that MS were obtained at short times (<30 s) and ripening took place to form ST NHDs for t > 2.5 min; (ii) O’Sullivan et al claimed that it was easy to achieve ST NHDs for t < 2 min; and (iii) Banin’s group ,, found that ST NHDs were obtained under much longer deposition times (>30 min) and at low temperature (<277 K). These different results suggest that the underlying growth mechanism remains to be investigated.…”

“…Most of the research concerning single-tip NHDs has been performed under the following conditions: low temperature, , long alkyl amine ligands, anaerobic conditions, or/and high reactivity gold precursors (AuCl 3 , AuCl), all of them mainly set to reduce the defect deposition, thereby improving the ratio of single-tip NHDs. Moreover, even if the presence of a hole scavenger has been shown to play a vital role in the photodeposition process of semiconductors (such as TiO 2 ), − it has rarely been mentioned as such in the photodeposition on a cadmium chalcogenide semiconductor. ,,, This is probably linked to the fact that even without the presence of a dedicated hole scavenger the chalcogenide anions forming the rods participate to the reduction of Au 3+ ions with a concomitant etching of the rods , or didodecyl dimethylammonium bromide (DDAB) and dodecylamine (DDA) a primary amine claimed to act not only as a reducing and stabilizing agent, but most probably also as a hole scavenger. ,, We should also mention that Kalisman et al commented on the necessary presence of a hole scavenger to favor the hydrogen yield during the photocatalytic reaction . Moreover, as far as we are aware, there is still no methodical research on the various parameters affecting the single-tip production, especially concerning the systematic variation of the ratios of single-tip (ST), double-tip (DT), and multiple-site (MS) hybrid nanoparticles (HNPs) even if the sites and size control have a great impact in catalysis and chirality .…”

“…Moreover, as far as we are aware, there is still no methodical research on the various parameters affecting the single-tip production, especially concerning the systematic variation of the ratios of single-tip (ST), double-tip (DT), and multiple-site (MS) hybrid nanoparticles (HNPs) even if the sites and size control have a great impact in catalysis and chirality . For instance, many reported data in the absence of dedicated hole scavenger (MeOH, EtOH, ...) gave deposition results which varied from each other ,,,,,,− and where the true effect of parameters such as the Au/DRs ratio and the role and ratio of hole scavenger were not specifically identified. The same statement can be made about the effects of light intensity when considering photodeposition.…”

Ligand-Induced Chirality in Asymmetric CdSe/CdS Nanostructures: A Close Look at Chiral Tadpoles

“…However, the demand from scientists to fundamentally extend the range of shape-controlled synthesis methods and to develop unique physicochemical properties for material designs has had considerable growth concerning to the great potential of these quantum materials in the areas of nanoscience, bionanotechnology and optoelectronics. [5][6][7][8][9][10][11][12][13][14] On the other hand, ligand-induced chirality in nanostructures such as metal nanocrystals (NCs) and clusters, [15][16][17][18][19] QDs, [20][21][22][23] carbon-based materials [24][25][26] and semiconductors [27][28][29][30][31][32][33] has revolutionized the concept of basic light-matter interactions into a chiroptics realm where more sophisticated phenomena such as circular dichroism (CD) and circularly polarized luminescence (CPL) are dominant. [34][35][36] With postsynthetic ligand exchanges, the optical chirality can be transferred to the NCs via ligand-metal interactions.…”