We modulate electron-hole separation at semiconductor junctions by stabilization of ‘non-native’ structures which are nanostructures whose discrete translational symmetry is different from that of the crystalline ground state. Four materials synthesis strategies involving click and colloidal chemistry are demonstrated and these methods can be potentially generalized to design a range of isomaterial and heteromaterial heterojunctions. The dynamics of electron-hole separation is studied using electrochemical impedance and photolumiscence spectroscopy.

Introduction

Enhancing electron-hole separation is critical in many optoelectronic devices and specifically in photoelectrochemical systems. The later involve three coupled physicochemical events: photon absorption, electron-hole separation and surface electrocatalysis. While heuristics for the optimization of photon absorption and surface electrocatalysis have been less challenging via trends in band-gap and “Volcano curve/Sabatier principle”, general strategies for increasing electron-hole separation has been less successful. In principle, based on the band-edge position of semiconductors, junctions can be formed to modulate electron-hole separation. The caveat behind this argument is the difficulty in forming an interface between two semiconductors using techniques that are relatively inexpensive and the present study addresses this issue. To expand the materials space, we have explored strategies for the stabilization of ‘non-native’ structures which are nanostructures having discrete translational symmetry in the sub-surface regions different from that present in the sub-surface regions of the thermodynamically most stable form of large crystals. These non-native structures have different physico-chemical properties (e.g. band-gap, band-edges and surface electrocatalysis) in comparison to bulk ‘native’ structure due to different chemical coordination. The problem of stabilization of lattice mismatched heterostructure is circumvented through ‘click’ chemistry and techniques from colloidal chemistry are used for assembling a range of interfaces.

Materials and Methods

Heterostructures are made through ‘click’ chemistry of alkyne-azide linkage. Heterostructures are attached to the conducting substrate using similar click reaction. Colloidal synthesis of crystals wherein there is pseudomorphic growth over a substrate is also utilized and morphology is controlled via a range of precursor conditions.

Results and Discussion

Native/Non-native semiconductor junctions are assembled via the following four approaches:

1) After obtaining non-native structure by variety of methods (for e.g. surface ligand control, external pressure, dopant), thermal driven arrested phase transition gives raise to isomaterial heterostructures.

2) The above strategy cannot be generalized to some systems due to surface energetics. To circumvent such limitations, heterostructures can be assembled via silica template mediated colloidal synthesis.

3) Pseudomorphic growth via colloidal synthesis by varying the precursor concentrations yields a variety of structures whose basis can be rationalized by interfacial energies along different Miller directions.

4) The most general method seems to be using click chemistry, which has been generalized to a variety of heterostructures of widely different lattice constants. The stability of clicked interfaces has been tested against a wide range of optoelectronic and electrochemical conditions. We have also clarified the nature electron transfer across the clicked interface.

The dynamics of electron-hole separation across the heterojunctions has been analyzed via electrochemical and photolumiscence spectroscopy.

Significance

Generic strategies potentially extendable to all kinds of semiconductor heterojunctions has been demonstrated. On-going efforts are towards modeling the exciton lifetime in the assembled heterostructures.