Effect of Pressure on Interband and Intraband Transition of Mercury Chalcogenide Quantum Dots

Abstract

Mercury chalcogenide nanocrystals generate a lot of interest as active materials for low cost infrared sensing. Device improvement requires building a deeper understanding of their electronic structure which combines inverted band ordering, quantum confinement and dependence to surface chemistry. This is particularly true with the development of mercury chalcogenide colloidal heterostructures (HgSe/HgTe, HgTe/CdS…). In this case the lattice mismatch induces a strain which affects significantly the band gap given the narrow band gap nature of the material. Here we study the effect of pressure on interband and intraband transitions in a series of HgTe and HgSe colloidal quantum dots. We demonstrate that in HgTe and HgSe, the nanocrystal morphology stabilizes the zinc blende phase up to 3 GPa. Under compressive strain, the interband signal blueshifts by 60 meV/GPa, while the intraband transition redshifts by a small amount (8 meV/GPa). Using an 8-band k•p formalism, we reveal that the interband shift has the same origin as the one observed for bulk material (change of effective mass, followed by band gap opening), while the intraband shift can be attributed to an increase of effective mass only.

The Strong Confinement Regime in HgTe Two-Dimensional Nanoplatelets

“…The confinement energy gradually increases with pressure corresponding to a positive value for dEG/dP. This is similar to what have been measured for HgTe NCs, 45 and wider band gap II-VI semiconductor quantum dots such as CdSe 49 and core shell made of CdSe/ZnS. 50,51 Applying pressure in the range of existence of the zinc blende phase can leads to a shift of the confinement energy up to 300 meV, which is far more than the shift induced by cooling the sample from room temperature to cryogenic condition (≈35 meV).…”

“…Similarly, we measured the pressure dependence of the band gap for the HgTe NPLs at various fixed temperatures. Note that for HgTe NCs the pressure dependence of the band gap was already reported in ref 45 at room temperature. This is why the effect of pressure is discussed in the case of NPL here.…”

“…Simulated confinement energy variation over =4 GPa pressure difference for 3 temperatures. The light orange dot is the experimental measurement around 150 K. The two red squares are the measurements from Ref45 at 300 K. c. Experimental confinement energy EG shift as a function of an applied hydrostatic pressure for HgTe NPLs for three different temperatures. d. Simulated confinement energy EG shifts as a function of an applied pressure for HgTe NPLs from 0K to 300K.The indicated slope corresponds to 0 KUsing −85 meV/GPa and +220 meV/GPa respectively for / and / , the (3+1) bands model can reproduce the pressure dependence of the confinement energy of the NPL, see Figure6c and d. Also note that using the temperature dependence determined previously, it is possible to reproduce the weak temperature dependence of the pressure potential / of the confinement energy in the case of NPL, see the four simulated temperatures in Figure6d, while capturing the moderate confinement dependence as depicted in Figure6band S13a.…”

Mercury Chalcogenide Quantum Dots: Material Perspective for Device Integration

“…248 The value of 1.4 GPa typically falls in the range of pressures that are induced by the growth of a shell on a core if the two material lattices are not matched. 249 Livache et al 240 estimated that the growth of a CdS shell on a HgTe core (10% lattice mismatch) should lead to a pressure of 2.6 GPa on the core, enough for the bulk to induce a structural phase transformation. Thus, by applying pressure on HgX NCs, it can be determined whether a change in the spectroscopic properties is the result of a phase change.…”

Direct Imprinting of Quasi‐3D Nanophotonic Structures into Colloidal Quantum‐Dot Devices

“…One general concern of patterning is the possible changes of interfacial chemistry and sintering. However, the pressure range used in imprinting is 20–70 MPa, which is two orders smaller than the pressures required for phase changes of lattice structures in CQDs . We first measured the resistance in situ during the loading and unloading process of pressure (Figure S11, Supporting Information).…”