Engineering the Surface Chemistry of Colloidal InP Quantum Dots for Charge Transport

Abstract

Colloidal InP quantum dots (QDs) have emerged as potential candidates for constructing nontoxic QD-based optoelectronic devices. However, charge transport in InP QD thin-film assemblies has been limitedly explored. Herein, we report the synthesis of ∼8 nm edge length (∼6.5 nm in height), tetrahedral InP QDs and study charge transport in thin films using the platform of the field-effect transistor (FET). We design a hybrid ligand-exchange strategy that combines solution-based exchange with S 2− and solid-state exchange with N 3 − to enhance interdot coupling and control the ndoping of InP QD films. Further modifying the QD surface with thin, thermally evaporated Se overlayers yields FETs with an average electron mobility of 0.45 cm 2 V −1 s −1 , ∼10 times that of previously reported devices, and a higher on−off current ratio of 10 3 −10 4 . Analytical measurements suggest lower trap-state densities and longer carrier lifetimes in the Se-modified InP QD films, giving rise to a four-time longer carrier diffusion length.

Synthesis, Surface Chemistry, and Fluorescent Properties of InP Quantum Dots

“…It may be that previous acid-free InP syntheses did not produce tetrahedral InP because they did not grow large enough to exhibit faceting. , Interestingly, the authors note that these structures exhibit a higher energy first exciton peak than similarly sized spherical InP QDs, indicating increased electron confinement. A more recent study by Zhao et al compared spherical and tetrahedral InP QDs of similar volumes and found that at small sizes (<75 nm 3 ) they exhibited similar exciton energies, but as particle volumes increased the tetrahedra maintained higher exciton energies while the spheres red-shifted significantly toward the bulk band gap . It should be noted that since spheres with diameter, d , contain more than 4 times the volume of tetrahedrons with the same edge length, d , InP QD samples with the same diameter and edge lengths should not be assumed to be the same size, especially when using sizing curves that assume spherical shape.…”

Development of Group III–V Colloidal Quantum Dots for Optoelectronic Applications

“…In general, the charge transport in the CQD film is assessed by measuring the mobility. ,, The reported carrier mobilities of group III–V CQD FETs are summarized in Table . A group III–V CQD film with record mobility (10 –2 –10 1 cm 2 /(V·s)) was achieved by chalcogenide passivation. − However, the resulting device often suffered from poor stability and undesired trap states, which limited its further use in various optoelectronic applications. , The development of strategies for fabricating conducting films is on course, where films fabricated using halide or short organic ligands ,, with or without fluoride pre-treatment exhibit a mobility of about 10 –2 –10 –5 cm 2 /(V·s). The modest conductivity of group III–V CQD films compared to films of other ionic CQDs or bulk materials may come from the high trap density in the former.…”

“…61−64 However, the resulting device often suffered from poor stability and undesired trap states, which limited its further use in various optoelectronic applications. 64,65 The development of strategies for fabricating conducting films is on course, where films fabricated using halide or short organic ligands 22,56,63 with or without fluoride pre-treatment 66 exhibit a mobility of about 10 −2 −10 −5 cm 2 /(V•s). The modest conductivity of group III−V CQD films compared to films of other ionic CQDs or bulk materials may come from the high trap density in the former.…”

“…58,68 Gas-phase deposition of chalcogenide atoms on the surface of CQDs is another prospectively powerful method that can increase the mobility with better surface passivation (Figure 4b). 63,69 Lastly, thin-shell passivation or shell coating with defect-tolerant materials might be the solution for effective trap passivation while maximizing charge transport.…”

“…Unless a short-ligand-based co-passivation strategy for handling fractional dangling bonds is developed, highly efficient thin-film device applications of group III–V CQDs seem elusive. Surface modification using metal halides can be effective as they can passivate the CQD surfaces as both X- and Z-type forms. , Gas-phase deposition of chalcogenide atoms on the surface of CQDs is another prospectively powerful method that can increase the mobility with better surface passivation (Figure b). , Lastly, thin-shell passivation or shell coating with defect-tolerant materials might be the solution for effective trap passivation while maximizing charge transport.…”