Pushing the Band Gap Envelope of Quasi-Type II Heterostructured Nanocrystals to Blue: ZnSe/ZnSe1-XTeX/ZnSe Spherical Quantum Wells

Quasi-type II heterostructured nanocrystals (NCs) have been of particular interest due to their great potential for controlling the interplay of charge carriers. However, the lack of material choices for quasi-type II NCs restricts the accessible emission wavelength from red to near-infrared (NIR), which hinders their use in light-emitting applications that demand a wide range of visible colors. Herein, we demonstrate a new class of quasi-type II nanoemitters formulated in ZnSe/ZnSe1-XTeX/ZnSe seed/spherical quantum well/shell heterostructures (SQWs) whose emission wavelength ranges from blue to orange. In a given geometry, ZnSe1-XTeX emissive layers grown between the ZnSe seed and the shell layer are strained to fit into the surrounding media, and thus, the lattice mismatch between ZnSe1-XTeX and ZnSe is effectively alleviated. In addition, composition of the ZnSe1-XTeX emissive layer and the dimension of the ZnSe shell layer are engineered to tailor the distribution and energy of electron and hole wave functions. Benefitting from the capabilities to tune the charge carriers on demand and to form defectfree heterojunctions, ZnSe/ZnSe1-XTeX/ZnSe/ZnS NCs show near-unity photoluminescence quantum yield (PLQY > 90%) in a broad range of emission wavelengths (peak PL from 450 nm to 600 nm). Finally, we exemplify dichromatic white NC-based light-emitting diodes (NC-LEDs) employing the mixed layer of blueand yellow-emitting ZnSe/ZnSe1-XTeX/ZnSe/ZnS SQW NCs.


Introduction
Colloidal semiconductor nanocrystals (NCs), in which charge carriers are confined in all three dimensions, have received tremendous interest as next-generation lightemissive materials owing to their unique optical properties such as broad absorption but narrow emission bandwidth and size-dependent band gap tunability across visible to near IR [1][2][3][4]. In addition to the quantum confinement effect, the heterojunction in NCs enables further control over the spatial distribution of charge carriers and their dynamics, boosting practicable use of NCs in displays [5][6][7], luminescent solar concentrators (LSCs) [8,9], and biomarkers [10]. The representative is the core/shell heterostructures, wherein the charge carriers in the core are decoupled from the surface trap states to award photochemical stability and near-unity luminescence efficiency at room temperature [11][12][13].
Among the core/shell heterostructures, NCs with the quasi-type II band alignment have been of particular interest due to their great potential for controlling the interactions among charge carriers. In quasi-type II heterostructured NCs, one charge carrier is confined in the core, whereas the other charge carrier is delocalized over the entire volume, thus controlling the geometry or the dimension of the core, and the shell allows managing the electron and hole wave functions independently. The distinguished feature of quasi-type II heterostructured NCs is the suppression of nonradiative Auger recombination processes of multicarriers (more than one electron and one hole) [14][15][16], which is the key to high efficiency light-emitting diodes and low threshold lasers.
To achieve quasi-type II NC emitters, two semiconductor materials having similar conduction band edge (CBE) energy levels or valence band edge (VBE) energy level need to form a contact without the interfacial defects. So far, reports on quasi-type II NCs have been limited to few cases (i.e., CdSe/CdS [15][16][17][18], PbSe/CdSe [19], and CuInS 2 /CdS [20,21]). The lack of materials choice for quasi-type II NCs restricts the accessible emission wavelength from red to NIR, which in return hinders their use in light-emitting applications that demand a wide range of visible colors.
Herein, we demonstrate a new class of quasi-type II nanoemitters formulated in ZnSe/ZnSe 1-X Te X /ZnSe seed/spherical quantum well/shell heterostructures (SQWs). The composition of the ZnSe 1-X Te X emissive layer and the dimension of the ZnSe shell layer are engineered to tailor the distribution and energy of hole and electron wave functions and their interactions. In addition, a comprehensive study across spectroscopic analysis, structural analysis, and calculation is conducted to assess the role of the composition and the geometry of given SQW NCs on their photophysical properties. Finally, we exemplify dichromatic white lightemitting diodes enabled by excellent luminescence efficiency of resulting SQW NCs in a range of emission colors from blue to orange.

Device Fabrication.
Before ZnSe/ZnSe 1-X Te X /ZnSe NC-LED fabrication, patterned indium tin oxide (ITO) glass substrates were cleaned with acetone, isopropyl alcohol, and deionized water for 15 min. First, ZnO NP solution (20 mg/ml in butanol) was spun-cast on an ITO substrate at a spin rate of 2000 rpm for 40 seconds followed by annealing at 100°C for 30 min. Afterwards, PFN solution (0.5 mg/ml in methanol/acetic acid mixed solvent) was spun-cast on the ZnO-coated substrates at a spin rate of 4000 rpm for 30 seconds. The PFN-coated substrates were dried for 30 minutes under vacuum condition. ZnSe/ZnSe 1-X Te X /ZnSe/ZnS NC solutions (10 mg/ml in toluene) were spun-cast at 4000 rpm for 30 seconds to form 2-3 monolayers of NCs. Finally, BCBP (60 nm), MoO 3 (10 nm), and Al (130 nm) layers were thermally evaporated under a vapor pressure of 1 × 10 −6 Torr. The deposition rates of each layer were 0.5-1 Å/s for BCBP, 0.1-0.2 Å/s for MoO 3 , and 3-5 Å/s for Al. The crystalline structures of NCs were investigated by synchrotron X-ray diffraction measurements (JP/D/MAX-2500H, X − ray wavelength = 1:5406 Å) The XRD data were acquired in the 2θ range between 20°and 60°at a scan rate of 1°/min. The XRD patterns of ZnSe and ZnSe 1-X Te X in ZnSe/ZnSe 1-X Te X NCs were decoupled by subtracting a separately measured XRD pattern of ZnSe NCs from that of ZnSe/ZnSe 1-X Te X NCs, considering the volume of each component.

Results and Discussion
As a new class of quasi-type II NCs, we construct heterojunctions made of ZnSe 1-X Te X emissive layer passivated by a ZnSe shell layer for the following reasons. First, the band gap of ZnSe 1-X Te X bulk semiconductors can cover a wide range of visible regions, from blue to orange, depending on the alloy compositions. In addition, the energy offset between conduction band edge energy levels (ECBE) of ZnSe and ZnSe 1-X Te X is similar to the thermal energy of room temperature (kBT = 25 meV), allowing the electron wave function to spread over the entire volume of NCs. Furthermore, the crystal structures of ZnSe and ZnTe are both zincblende, and thus, the epitaxial growth of ZnSe on ZnSe 1-X Te X is achievable.
Heterostructuring ZnSe 1-X Te X and ZnSe in a conventional core/shell geometry entails the nucleation of the ZnSe 1-X Te X emissive core and the epitaxial growth of the ZnSe shell, but both appear to be challenging to be experimentally realized via wet-chemistry. First, the significant reactivity difference between Se and Te precursors often results in the independent nucleation of ZnSe and ZnTe instead of the formation of a homogeneous ZnSe 1-X Te X core, particularly at high Te contents (X ≥ 0:30) ( Figure S1). In addition, the large lattice mismatch between the ZnSe 1-X Te X emissive core and the ZnSe shell promotes the creation of misfit defects at the interface, which gives rise to the reduction of PL QYs with increasing shell thicknesses [22]. The difficulties in the synthesis of homogenous ZnSe 1-X Te X emissive cores and defect-free growth of the ZnSe shell on ZnSe 1-X Te X impedes the realization of high quality ZnSe 1-X Te X /ZnSe core/shell heterostructured NCs with varying Te contents [23][24][25].
As a mean to relieve the structural stress between the ZnSe 1-X Te X emissive layer and the ZnSe shell, we adopt the concept of coherently strained multilayered heterostructures [26]. Specifically, we design the ZnSe/ZnSe 1-X Te X /ZnSe seed/spherical quantum well/shell heterostructures (Figure 1(a)), in which the ZnSe 1-X Te X emissive layer is successively grown on top of the ZnSe seed and passivated by the ZnSe shell layer. In a SQW geometry, the sandwiched emissive layer is strained by the presence of an underlying seed to relieve the structural stress against the shell layer, enabling to attain high luminescence efficiency even with thick shells [26,27]. Besides, the growth of the ZnSe 1-X Te X emissive layer requires the injection of mixed Se and Te precursors at the concentration below the critical concentration for nucleation of ZnSe and ZnTe, which allows to avoid the problems that originated from the inhomogeneity in size and composition among NCs.
As a first step, we optimize the reaction condition for the synthesis of ZnSe seeds with a radius (r) of 0.6 nm and further ZnSe growth up to 1.8 nm ( Figure S2). At the given homoepitaxy growth condition, we replace in part the Se precursor with the Te precursor to grow the ZnSe 1-X Te X emissive layer. Resulting NCs show uniform size distribution (std~10%) and single Gaussian PL emission regardless of Te contents. Small particles or side PL peaks, which are the signs of homogenous nucleation of ZnSe or ZnTe, do not appear during the growth ( Figure S3). The composition analysis in chorus with the size and shape monitoring confirms that the Te content (X) of the ZnSe 1-X Te X emissive layer remains unchanged along the radial direction ( Figure S4). These results coherently attest to the successive growth of homogeneous ZnSe 1-X Te X emissive layers on the surface of ZnSe seeds. The sequential growth of ZnSe on top of ZnSe/ZnSe 1-X Te X NCs at the given reaction condition results in ZnSe/ZnSe 1-X Te X /ZnSe SQW NCs. Figure 1 shows the structure and optical properties of the resulting ZnSe/ZnSe 1-X Te X /ZnSe SQW NCs. In such a geometry, the hole is strongly confined within the ZnSe 1-X Te X emissive layer, whereas the electron is delocalized over the entire volume of the ZnSe/ZnSe 1-X Te X /ZnSe SQW NCs (Figure 1(c)). The dimensions of the ZnSe seed (radius, r) and the ZnSe 1-X Te X emissive layer (thickness, l) are fixed to be r = 0:6 nm and l = 1:2 nm for all samples, and thus, the variation in the ZnSe shell thickness (h) determines the distribution of the electron wave function and its energy. By contrast, the hole wave function is solely affected by the composition of the ZnSe 1-X Te X emissive layer (X). Engineering these parameters permits to tune the emission wavelength of heterostructured NCs from blue (peak PL at 450 nm) to orange (peak PL at 600 nm) (Figures 1(b) and 1(d)).
A notable feature of the resulting NCs is the excellence in PL QYs. Specifically, passivating additional thin ZnS shell layer (0.6 nm) on the exterior of ZnSe/ZnSe 1-X Te X /ZnSe SQW NCs assists in diminishing the influence of surface trap states on the charge carrier wave functions, resulting in over 90% PL QY of NCs even in cases with high Te content (Figure 1(d) inset). The near-unity PL QYs for ZnSe/ZnSe 1-X Te X /ZnSe/ZnS SQW NCs clearly contrast with previous reports on ZnSe 1-X Te X /ZnSe/ZnS core/shell NCs with similar structural parameters (PL QY = 52% for X = 0:02, ZnSe shell thickness = 1:5 nm, and ZnS shell thickness = 0:4 nm) [22], which in turn implies that the formation of misfit defects at the interface between the ZnSe 1-X Te X emissive layer and ZnSe surround media is effectively suppressed in the SQW platform.
To validate the impact of the geometry on the lattice strain relaxation between ZnSe 1-X Te X and ZnSe, we characterize the lattice constant of ZnSe 1-X Te X emissive layers grown on ZnSe seeds (Figure 2). We obtain the X-ray diffraction patterns of ZnSe/ZnSe 1-X Te X seed/spherical quantum well samples and decouple the contribution of ZnSe seeds to extract the patterns of ZnSe 1-X Te X only (see Section 2.5). ZnSe 1-X Te X films are well-known to follow Vegard's law in  Energy Material Advances a wide range of alloy ratios (0 < X < 0:9) when grown in bulk films [28]. The diffraction patterns of ZnSe 1-X Te X emissive layers clearly deviate from the lattice constant estimated from Vegard's law (Figure 2(b)), indicating that the ZnSe 1-X Te X spherical quantum well layers are strained to fit into the ZnSe sublayer. Specifically, in the SQW platform, the lattice mismatch between ZnSe and ZnSe 0.8 Te 0.2 emissive layer is suppressed by 55%, 49%, and 23% along (111), (220), and (311) directions compared to the bulk parameters, respectively. The reduction in the lattice mismatch between ZnSe and ZnSe 1-X Te X alleviates the lattice stresses at the interface between ZnSe and ZnSe 1-X Te X and aids to extend the critical thickness for the defect formation during the SQW NC growth, allowing ZnSe/ZnSe 1-X Te X /ZnSe SQW NCs to reach near-unity PL QYs. The achievable emission window of ZnSe/ZnSe 1-X Te X /ZnSe SQW NCs by varying the composition of ZnSe 1-X Te X and the dimension ranges from blue (450 nm, 2.76 eV) to orange (600 nm, 2.07 eV), which exceeds that of bulk ZnTe (E g,ZnTe = 2:3 eV) (Figure 3(a) and Figure S5). The wideranging band gap tunability is attributed to the unusual strong band gap bowing of ZnSe 1-X Te X [29], which is given by   Energy Material Advances where X denotes the Te content in ZnSe 1-X Te X and b ZnSe 1-X Te X is the bowing parameter (1.45 eV) [29]. We note that the band gap bowing parameters of ZnSe 1-X Te X is substantially large compared to the cases with ZnSe 1-X S X or Cd 1-X Zn X Se alloys (b ZnSe 1-X S X = 0:41 eV, b Cd 1-X Zn X Se = 0:35 eV) [30]. The strong band gap bowing could stand for the strong bowing in either the conduction band edge energy level (E CBE ) or valence band edge energy level (E VBE ) of ZnSe 1-X Te X or a combination of both, whose impact to the distribution of electron and hole wave functions and their recombination processes varies significantly. So far, the key parameters (e.g., the band position and the effective masses of charge carriers (m e * , m h * )) for ZnSe 1-X Te X alloys are largely missing. To gain the band position of ZnSe 1-X Te X alloy NCs, we conduct spectroscopic analysis on ZnSe/ZnSe 1-X Te X /ZnSe SQW NCs in parallel with simulation on bulk ZnSe 1-X Te X alloys (Figures 3(a) and 3(b) and Table S1). The ultraviolet photoemission spectroscopy (UPS) on ZnSe/ZnSe 1-X Te X /ZnSe SQW NCs with varying Te ratios allows to measure the quantized energy state of hole (1S h ), and the UV-Vis spectroscopy enables estimating the quantized energy state of electron (1S e ) (Figure 3(b) and Figure S6). The quantum mechanical calculation in ZnSe/ZnSe 1-X Te X /ZnSe SQW NCs allows us to approximate energy positions (E CBE and E VBE ) and effective masses of charge carriers for ZnSe 1-X Te X alloys with varying Te contents. The experimental results show that the unusual band gap bowing of ZnSe 1-X Te X alloys is a result of the strong bowing of E VBE of ZnSe 1-X Te X alloys. In fact, E CBE of ZnSe 1-X Te X alloys varies linearly along with Te contents. Figure 3(c) displays the overlap integral of ZnSe (r = 0:6 nm)/ZnSe 1-X Te X (l = 1:2 nm)/ZnSe SQW NCs as a function of the Te ratio (0 ≤ X ≤ 0:5) and the ZnSe shell thickness (0 ≤ h ≤ 3:0 nm). The asymmetric changes in E CBE and E VBE of ZnSe 1-X Te X alloys allow constructing quasitype II band alignment across the ZnSe seed, ZnSe 1-X Te X emissive layer, and ZnSe shell layer, in which the electron wave function is delocalized over the entire volume, whereas the hole wave function is strongly confined within the ZnSe 1-X Te X emissive layer (Figure 3(d)). In a given ZnSe (r = 0:6 nm )/ZnSe 1-X Te X (l = 1:2 nm)/ZnSe geometry, the quantum confinement effect is only valid to electron wave functions and the energy of hole wave function is primarily governed by the composition of the ZnSe 1-X Te X emissive layer. The chosen variation in the composition (X) of the ZnSe 1-X Te X emissive layer and the ZnSe shell thickness (h) modulates the energy levels of 1S h and the 1S e of NCs by 0.5 eV and 0.2 eV, respectively, and the combination of both enables to engineer the optical band gap of quasi-type II NCs from 2.07 eV to 2.76 eV.
The electronic structure of given NCs is similar to the case of quasi-type II heterostructures made of CdSe and CdS, but the variation in electron and hole overlap integral appears to be more dramatic than the case with CdSe and CdS [15]. For example, the growth of the ZnSe shell from 0.6 nm to 3.0 nm reduces the electron and hole overlap integral of ZnSe (r = 0:6 nm)/ZnSe 0.67 Te 0.33 (l = 1:2 nm)/ZnSe SQW NCs from 0.9 to 0.2, which accompanies the substantial change in the single exciton recombination rate from 0.0161 ns -1 to 0.0064 ns -1 (Figure 3(e)). The wider window to modulate the electron and hole wave functions is important not only because it provides optical band gap tunability of quasi-type II NCs to complement with that of previously reported quasi-type II NCs [15][16][17][18][19][20][21], but also because it promises the capability to engineer the multicarrier dynamics of NCs that is the key for realizing nonblinking quantum emitters or high power light sources. Indeed, trion lifetime measurements ( Figure S8) indicate that multicarrier dynamics in ZnSe (r = 0:6 nm)/ZnSe 0.67 Te 0.33 (l = 1:2 nm )/ZnSe/ZnS SQWs are similar to those measured in CdS/CdSe/CdS SQWs [27] and about one order of magnitude slower than the ones reported for regular type I core/shell quantum dots [31].

Summary
We have demonstrated highly luminescent quasi-type II heterostructured nanoemitters formulated in ZnSe/ZnSe 1-X Te X /ZnSe seed/spherical quantum well/shell heterostructures. In a given formation of NCs, the electron wave function is delocalized over the entire volume, whereas the hole wave function is strongly confined within the ZnSe 1-X Te X emissive layer, and thus, the composition of the ZnSe 1-X Te X emissive layer and the dimension of the ZnSe shell layer permit to tailor the hole and electron wave function independently. The SQW structural platform alleviates large lattice mismatch between the ZnSe 1-X Te X emissive layer and the ZnSe passivation layer. Benefitting from the capabilities to tune the charge carriers on demand and to form defect-free heterojunctions, resulting ZnSe/ZnSe 1-X Te X /ZnSe/ZnS SQW NCs show near unity photoluminescence quantum yield (PL QY > 90%) in a range of emission wavelengths (peak PL from 450 nm to 600 nm), which have not been reached previously. Finally, we exemplify the use of ZnSe/ZnSe 1-X Te X /ZnSe/ZnS NCs as the emitters for white light sources.
The supplement of emission wavelengths achieved in the present study completes the emission window for the quasitype II NCs to cover the entire visible region. The apparent next step is to investigate the multicarrier dynamics of ZnSe/ZnSe 1-X Te X /ZnSe/ZnS SQW NCs in relationship with their structural formulation. In parallel, the delicate design of heterostructures by means of advances in wet-chemistry is required to engineer the multicarrier dynamics in give NCs. These all together will allow engineering the interplay of charge carriers of quasi-type II NCs on demand, hence fostering the use of quasi-type II NCs in a range of lightemitting applications including nonblinking quantum emitters, high power LEDs, or low threshold lasers.