Introduction of II-VI, III-V, and I-III-VI Semiconducting QDs

In type II core/shell, two band gaps are staggered band, resulting in two spatially dissociated wavefunctions. The energy from radiative decay is determined by the difference between spatially different energy levels, which is smaller than the band gap of the core or shell. This difference separates the hole and the electron in different regions in this structure. This band gap structure enables the potential photovoltaic applications of such materials.^[52]

1.2.4 Introduction of II-VI, III-V, and I-III-VI Semiconducting QDs

Many QD materials composed of II-VI (e.g., CdTe), III–V (e.g., InP), and I-III-VI (e.g., CuInS2 or CIS) have also been investigated. Figure 1-8 shows that the core material determines the range of QD emission colors that can be tuned by NC size and with different materials emitting across portions of the UV, visible, and near-infrared (NIR) spectra.^[35,53,54] Therefore, this study describes the development in the areas of fundamental concepts and properties, which are then applied to LEDs and bioimaging.

Figure 1-8. (a) PL ranges of several semiconductors with various sizes[35]; (b) PL ranges of composition control and doping metal sulfide QDs.^[54]

(b) (a)

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1.2.4.1 QDs of Group BinaryII-VI

The II-VI compound semiconductors are composed of Zn²⁺, Cd²⁺, and Hg²⁺ with O^2-, S^2-, Se^2-, Te^2-, such as CdSe^[55], CdTe^[56], HgTe^[57], and ZnSe.^[58] These semiconducting QDs typically crystallize in either face-centered cubic (zinc blende) or hexagonal (wurtzite) crystal structures. The II-VI compound semiconductors can exhibit very good luminescence because they possess a direct band gap. The PL bands of these compound semiconductors are situated typically close to the absorption thresholds and sufficiently narrow full width at half-maximum (FWHM) from 35 nm to 60 nm with increasing particle size. Grabolle et al.^[59] improved the PL-QY of CdTe from 40% up to 60% with the use of thiol ligand.

Band gaps can be engineered by alloying the core.^[60-65] Optoelectronic properties of QDs can be controlled by manipulating the composition of the core materials and the ratio of the alloying materials. Emission colors of alloyed Zn1-xCdxS1-ySey across portions of the near-UV, visible, and NIR spectra can be tuned by various compositions (Fig. 1-9).^[67] These QDs are fabricated in QD-LEDs with high efficiency. Alloys of CdHgTe QDs exhibit NIR emission (600 nm to 1350 nm) by varying the stoichiometric ratio of two binary semiconductors.^[68-71] These QDs are also used in many applications including biological imaging.^[72-74]

Figure 1-9. Photograph of Zn1-xCdxS1-ySey QDs with varying sizes and compositions under UV light.^[67]

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1.2.4.2 QDs of Group BinaryIII-V

Studies on semiconducting NCs have focused on the fabrication of versatile groups II-VI QDs because of their potential application in lasers^[75], LEDs^[76], and biological studies.^[1,77,78] However, the presence of highly intrinsic toxic Cd limits their biological application. Therefore, overcoating the Cd-based core with a less toxic shell, such as ZnS, has been introduced to overcome this toxicity problem. The shell can prevent the release of the toxic Cd cations. However, this shell cannot ensure complete non-toxicity of II–VI QDs under UV light or oxidation. Cd cations can be released under such conditions by surface oxidation.^[79]

Semiconducting III-V QDs have been investigated in the past decade, particularly InP QDs, which possess a band gap of 1.35 eV. The principal attraction to these semiconductors focuses on the robustness of the covalent bond in groups III–V semiconductors compared with the ionic bond in the groups II–VI semiconductors. The formation of covalent bond enhances the optical stability of the QD systems. Thus, the reduction of the toxicity derived from the non-corrosive composition elements enables the use of QD systems in biological field.^[81,82] The resultant defects, also called surface sites, act as traps for non-radiative decay of the QDs under the excited state.^[83] Some of the excited electrons can cross to the surface states located in the intra band gap, and then recombine nonradiatively with the holes in the valence band, thereby decreasing the PL-QY. Several syntheses have been developed to enhance the PL-QY. To passivate surface defects, an inorganic shell made from large-band gap material is epitaxially grown around each core QD, resulting in an improved PL-QY and enhanced photostability.^[84-86] Lim et al.^[84] reported that the InP/ZnSeS core/shell exhibits the highest PL-QY, demonstrating that surface traps are effectively depressed after epitaxially depositing ZnSeS onto the InP bare core. The type and thickness of the inorganic shell are critical in tailoring the optical and electronic properties of QDs.^[87,

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88] In addition, chemically eliminating the particle surface of the InP QD core entails etching them with dilute ethanolic or butanolic solutions of HF ^[89,90] or NH4F.^[91] Liu et al.^[92] demonstrated a colloidal synthesis of good quality InP QDs using PCl3, which was reduced to elemental P with a superhydride solution. The QY of the resulting InP QDs reached only 0.25%, but was substantially increased to ∼20% after an appropriate HF-based photo-etching treatment. The FWHM of PL emission for typical InP QDs is broader (50 nm to 80 nm) than that of CdSe (15 nm to 40 nm), which induced a worse color saturation for InP compared with CdSe QDs. Yang et al.^[88] synthesized InP/ZnS NCs with tunable emission wavelength by changing the InP:ZnS without multiple injections, achieving an impressive FWHM of PL (38 nm) emission is achieved (Fig.

1-10). Obtaining InP QDs with high optical performance and pure color as good as CdSe QDs remains a remarkable challenge.

Figure 1-10. (a) Photograph of samples with different InP:ZnS ratios under UV light.

(b) PL-QY and FWHM of InP/ZnS NCs with various ratios of InP:ZnS.^[88]

(a)

(b)

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1.2.4.3 QDs of Group Ternary I-III-VI

Ternary I-III-VI QDs have attracted less research interest compared with binary III-V compounds (InP). The I-III-VI semiconductors composed of groups I (Cu, Ag), III (Al, Ga, In, Tl), and VI (S, Se, Te) elements can be conceptually derived from II-VI binary compounds by replacing two divalent cations with one monovalent and one trivalent cation; these elements are potentially less toxic. These compounds have a wide range of optical and electronic properties, depending upon each other for their chalcopyrite structure (Fig. 1-11).^[55]

Figure 1-11. (a) Schematic of the structural relationship among groups IV, II–VI, and I-III-VI2 semiconducting materials and (b) optical band gap of different chalcopyrite-type I-III-VI2 semiconducting QDs.^[55]

The development of synthesis methods for I-III-VI NCs with controlled size, shape, composition, and surface chemistry has attracted significant attention in the past five years (2008-2013), showing an exponential increase in publications. In particular, CIS, CuInSe2, and AgInS2 (AIS) have been intensively investigated.^[93-95]

(a)

(b)

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The I-III-VI QDs exhibit tunable spectroscopic properties from the NIR region, through visible, up to the UV region^[55], as well as other features, including large Stokes shifts (i.e., the energy difference between absorbance band gap and emission peak energy), long PL lifetime, and low toxicity^[96-99]. Elucidating the luminescence mechanism, understanding the factors affecting luminescence properties, and optimizing the luminescence efficiency of I-III-VI QDs are necessary. The basic quantum confinement effects in I-III-VI NCs provide size-tunable luminescent properties.^[55,98] However, the fluorescence feature of the I-III-VI QDs is entirely different from those of the II-VI or III-V QDs. [17, 88,100]

1.2.4.3.1 Donor-Acceptor Pair (DAP) Recombination

The FWHM of the PL emissions for these CIS QDs are approximately 90 nm to 120 nm. Large differences in absorption between broad emission bands in energy (Stokes shift) are typically observed in I-III-VI QDs. Radiative recombination of CIS QDs is associated with Cu deficiency. These results suggest the excitation of the electron-hole pair within intra-band gap, which is commonly referred to as the DAP recombination.[97,99,101] The electronic levels of one acceptor (VCu: Cu vacancy) and two donors (VS: S vacancy, InCu: Cu site was substituted by In) in bulk CIS, marked within the 1.53 eV band gap.^[55] The quantized CIS QDs of the electron [1S(e)] and hole [1S(h)]

levels are widened by quantum confinement effect^[102], and the donor and acceptor levels shift toward conduction band (CB) and valence band (VB) edges, respectively.

Bulk CIS can be formed from InCu-VCu recombination, resulting in NIR emission.

However, dominant DAP recombination pathway can be VS-VCu instead of InCu-VCu in CIS QDs (Fig. 1-12).^[101]

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Figure 1-12. (a) Schematic of the electronic energy levels of the donor (VS, InCu) and the acceptor (VCu) states in In-rich CIS bulk versus QD, proposing that VS and VCu levels can be moved along band gap widening. The recombination pathways designated with (i) and (ii) correspond to DAP (VS-VCu) and CB-VCu transitions for CIS core and CIS/ZnS core/shell QDs, respectively. ^[101]

1.2.4.3.2 Composition Tuning

The emission color can be tuned by adjusting the [Cu]/[In] or [Ag]/[In]

stoichiometry of CIS, CuInSe2, and AIS QDs. The blueshift in emission maximum with increasing Cu or Ag deficiency is attributed to the energy gap widening associated with the decrease in Cu or Ag content. This phenomenon is attributed to the maximum valence band level of CIS or AIS; this level consists of hybrid orbitals of S 3p and Cu 4d or Ag 5d [101,103,104], which reduces the number of orbitals originating from Cu or Ag, lowering the maximum valence band level, and thereby widening the band gap of CIS or AIS QDs. Improvement in PL QY is attributed to an increase of the population of DAP defects required for DAP recombination of excited charge carriers. Optimal

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QYs are obtained from CIS or AIS QDs that were synthesized with identical initial [Cu]/[In] or [Ag]/[In] molar ratios of 1/4.^{[99, 106]}

Adapting the nucleation- and diffusion-controlled alloying composition tuning allows band gap to be tuned for specific ranges and improves the PL-QY of QDs. For example, the introduction of Zn or Ga cation into CIS or AIS QD systems have been well-investigated to tune the absorption optical band gap and the emission spectra, overlapping the entire visible spectrum.^[106-110] Several studies have also exhibited that ZnS-AIS and ZnS-CIS-alloyed NCs also exhibit enhanced PL intensity in Fig. 1-13(a).

1.2.4.3.3 Surface Tuning

ZnS was epitaxially grown on CIS core QDs to form CIS/ZnS QDs. The resulting blueshift is believed to occur by cationic interdiffusion between the CIS core and the relatively wide band gap of the ZnS shell (bulk Eg = 3.68 eV), as shown in Fig. 1-13(b).

A considerable increase in PL-QY and material stability of I-III-VI NCs is evident upon surface coating of ZnS or CdS layers because of the elimination of surface trap states (dangling bonds). ^[97-99]

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