Ligand‐Induced Control of Photoconductive Gain and Doping in a Hybrid Graphene–Quantum Dot Transistor

Abstract

concentration can be modifi ed, and photoresponsivity of SLG can be signifi cantly enhanced by the choice of ligands. By reducing the length of capping ligands, hence the thickness of the dielectric barrier between the QDs and the SLG, and by preserving the integrity of the ligand layer, we achieve the efficient transfer of photogenerated carriers from the QDs to the graphene before recombining, resulting in enhanced photoresponsivities of up to ≈10 9 A W −1 .Single layer graphene grown by low-pressure chemical vapor deposition (LP-CVD) [ 14,15 ] was processed into a planar transistor and deposited onto a SiO 2 /n-Si substrate with oxide layer thickness t = 300 nm, as shown schematically in Figure 1 a (see also the Supporting Information, Figure S1). The length, L , and width, w , of the graphene channel are 6 and 10 µm, respectively. The n-Si layer serves as a gate electrode. Colloidal quantum dots with an average PbS core diameter of d = 4.5nm QD (Figure 1 b) were stabilized using the following capping ligands of different length, l , which determines the effective separation between the PbS nanocrystals and graphene: polyethylene glycol H (O CH 2 CH 2 ) n OH with n = 2000 for QD p2000 and n = 500 for QD p500 [ 16 ] and corresponding length of l ≈ 10 nm and l ≈ 5 nm, respectively; [ 17 ] a mixture of thioglycerol (TGL) and 2,3-dimercapto-1-propanol (DTG), l ≈ 0.5 nm for QD TGL , [ 18 ] see Table 1 . We note that the ligand size is estimated for extended molecules and does not include any effects that could arise from compression of the ligand layer upon drying under vacuum. The interparticle distances observed on TEM images are comparable with the theoretical estimate of the ligand length (see the Supporting Information, Figure S2). The QDs were dropcast from aqueous solution (5 mg mL −1 ) to produce continuous thick (>100 nm) coverage of the graphene layer, followed by drying for 12 h in vacuum at room temperature. The size of the QDs and their room temperature photoluminescence spectra (with a peak at photon energy ≈1.1eV) are similar in all three structures, i.e., they are not affected by the ligand. All measured devices demonstrate stable and reproducible behavior during the measurement period of at least 14 days. All samples were stored in dry nitrogen cabinet and all electrical measurements were performed at room temperature T = 300 K in vacuum (≈10 −6 mbar). The I V ( ) g characteristics were measured using slow sweeping rate, r , of the gate voltage, i.e., r ≤ 0.02 V s −1 .The pristine monolayer graphene devices show a linear dependence of current, I , on bias voltage, V s , and a minimum conductance at a gate voltage, V ≈ +30V g min corresponding to p-type doping with hole concentration, p ≈ 2 × 10 12 cm −2

High-response hybrid quantum dots- 2D conductor phototransistors: recent progress and perspectives

“…Ligand exchange processes are commonly used to replace the long-chain ligand with small molecules such as amines [32], thiols [33][34][35], and hydrazine [36]. However, it can be difficult to stabilize QDs such as PbS and PbSe using short surfactant ligands, as both size and shape of the dots can be compromised during the exchange process [35][36][37][38][39][40][41].…”

“…However, it can be difficult to stabilize QDs such as PbS and PbSe using short surfactant ligands, as both size and shape of the dots can be compromised during the exchange process [35][36][37][38][39][40][41]. In their work on QD solids, Jeong et al demonstrated complete removal of oleic acid on PbS QDs using ethanedithiol (EDT) and mercaptocarboxylic acids (MPA) treatments to fabricate QD solids with enhanced mobility and lifetime [33].…”

“…Meanwhile, Talapin et al completely removed the oleic acid on PbSe QDs with diluted hydrazine to fabricate phototransistors with high conductivities [35]. Turyanska et al also demonstrated significant enhancement in conductivity using short thiol-based ligands of thioglycerol/dithioglycerol or polyethylene glycol [36]. On the other hand, to achieve better control over the stoichiometry of the QDs during ligand exchange processes, Zhang et al demonstrated the use of a metalfree chalcogenide compound, (NH 4 )S 2 , to remove bulky surfactant ligands [37].…”

“…Dependence of the photoresponse of hybrid transistors on QD functionalization (capping with various ligands) was investigated in Refs. [36] and [67]. All hybrid phototransistors demonstrated high gain, responsivity, and detectivity but relatively long rise and relaxation times, which changes from several subseconds to dozens of seconds.…”

“…Turyanska et al in Ref. [36] investigated PbS QDs of 4.5-nm diameter capped by polyethylene glycol H-(O-CH 2 -CH 2 ) n -OH with n = 2000 (P2000) and n = 500 (P500). The P2000 and P500 ligands have length of 10 nm and 5 nm correspondingly.…”