Controlling potential landscape of heterostructured nanocrystals with interfacial polarization

: The potential profile and the energy level offset of nanocrystals ( h -NCs) determine the photophysical properties and the charge transport characteristics of h -NC solids. However, limited material choices for heavy metal-free III-V/II-VI h -NCs pose challenges in comprehensive control of the potential profile. Herein, we present an unprecedented approach to such control by steering dipole moments at the interface of III-V/II-VI h -NCs. The controllable heterovalency at the interface is responsible for interfacial dipole moments that result in the vacuum-level shift, providing an additional knob for the control of optical and electrical characteristics of h -NCs. We capitalize on the atomic precision with which to synthesize h -NCs by correlating interfacial dipole moments to photochemical stability and optoelectronic performance of resulting h -NCs.

4 between III-V core and II-VI shell. First, we examine the role of heterovalent bonds (III-VI and II-V bonds) at the interface of III-V/II-VI h-NCs as the source of interfacial electric dipoles that modulate the potential profile of h-NCs and the electronic energy levels. The cation exchange process at III-V/II-VI heterovalent interface results in the change in the interfacial polarization.
Finally, we discuss the impact of engineered interfacial dipoles on the transport characteristics of charges across h-NC and surrounding media and thus on the performance of end-applications utilizing optical and optoelectronic properties of III-V/II-VI h-NCs. There are two types of heterovalent bonds (III-VI and II-V bonds) that construct dipole moments at the interface of III-V/II-VI h-NCs. Fig. 1 exemplifies the case of zinc blende (ZB) InP/ZnSe core/shell h-NCs. X-ray photoelectron spectroscopy (XPS) reveals that the In-Se bonding at the interface increases the binding energy of In3d5/2 but decreases the binding energy of Se3d5/2 ( Fig. 1b, Supplementary Fig. 1), reciting the obvious notion that indium is partially positively charged (δ + ) and selenium is partially negatively charged (δ -) to create a bond dipole moment pointing from In to Se. In the same manner, Zn-P bonding yields a bond dipole moment pointing from Zn to P (Fig. 1b, Supplementary Fig. 1). The vector sum of bond dipole moments from In-Se bonds and Zn-P bonds constructs the net dipole moments at the interfaces. For The difference of dipole moments produced by In-Se bonds and dipole moments by Zn-P bonds creates a net electric field at the interface 29,30 that gives rise to the change in the potential profile of InP/ZnSe h-NCs (Fig. 1c). In general, the outward dipole moment lowers the potential of InP core, whereas the inward dipole moment elevates it. The vacuum-level shift of the core 6 alters the potential profile between the III-V core and the II-VI shell, which consequently redefines the electronic energy level and the spatial distribution of charge carriers.
The magnitude of the vacuum-level shift (ΔEVLS) resulting from the interfacial dipole moments is given by where n is the total number of bond dipole at the interface, ⊥ is the dipole moments normal to the interface, A is the interfacial area, κ is the average dielectric constant of the interfacial dipole layers, and 0 is permittivity of free space 31 . In InP/ZnSe core/shell h-NCs, the total number of In-Se and Zn-P bonds is determined by the contact area, the atomic In:P ratio in InP core is directly linked to the magnitude and the direction of interfacial dipole moments. Here, we   The energy shift of conduction energy levels is more significantly affected by the polarization than that of valence energy levels because of the smaller effective mass of electrons (me/m0 = 0.077 vs. mh/m0 = 0.56 for InP) 25 . For example, the electron confinement energy is 70 meV higher for InP/ZnSe h-NCs with ρ = 1.50 nm -2 than that with ρ = 0.09 nm -2 , whereas the difference of hole confinement energy is relatively small (20 meV), resulting in the substantial modification in the optical bandgap and photoluminescence among h-NCs constructed in the same dimension and geometry (Fig. 2c, d). In addition, due to the small effective mass, the electron wavefunction is more likely to leak into the ZnSe shell upon the reduction of the 9 potential barrier, while the hole wavefunction remains confined within the InP core ( Supplementary Fig. 4). The asymmetric changes of charge carriers result in the reduction of the overlap integral of electron and hole wavefunctions (Θe-h) and consequently the decrease in their radiative recombination rates (krad) (Fig. 2e). Regardless of the dipole density, InP/ZnSe h-NCs show near-unity photoluminescence quantum yields (PL QYs) (> 90 %) and retain their photophysical properties for an extended period, implying that the chosen structural variation at the interface does not necessarily impair the structural stability of h-NCs. The atomic composition at the heterovalent interface is directly linked to the interfacial dipole density that reshapes the potential profile of III-V/II-VI h-NCs. As a mean to control the atomic ratio, we synthesize III-V NCs with group III element-rich surfaces and foster the cation exchange reaction at the surface prior to the passivation of II-VI shells. proportional to the interfacial dipole density (ρ) (Fig. 3b).
These results coherently attest that, in addition to the quantum confinement effect, the   Table 5). It is noteworthy that, given the outcoupling efficiency of 20%, the obtained device efficiency is close to the theoretical limit.
We   coupled with a source meter unit (SMU 238, Keithley instruments).

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The authors declare that all data supporting this work are contained in graphics displayed in the main text or in supplementary information. Correspondence and requests for materials should be addressed to corresponding authors.