Effect of ligand self-assembly on nanostructure and carrier transport behaviour in CdSe quantum dots

Materials Chemistry and Physics 148 (2014) 253e261

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Effect of ligand self-assembly on nanostructure and carrier transport behaviour in CdSe quantum dots Kuiying Li*, Zhenjie Xue State Key Laboratory of Metastable Materials Manufacture Technology & Science, Yanshan University, Qinhuangdao 066004, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


CdSe QDs modified by L-cysteine possess wurtzite nanocrystalline structures.
Carboxyl end groups in the ligand serve to increase the SPV response of CdSe QDs.
Terminal hydroxyl group in the ligand might accommodate nonradiative de-excitation process in CdSe QDs.
Increased length of the alkyl chains and side-chain radicals in the ligands partially inhibit carriers transport of CdSe QDs.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 February 2014 Received in revised form 10 June 2014 Accepted 24 July 2014 Available online 19 August 2014

Adjustment of the nanostructure and carrier behaviour of CdSe quantum dots (QDs) by varying the ligands used during QD synthesis enables the design of specific quantum devices via a self-assembly process of the QD coreeshell structure without additional technologies. Surface photovoltaic (SPV) technology supplemented by X-ray diffractometry and infrared absorption spectroscopy were used to probe the characteristics of these QDs. Our study reveals that while CdSe QDs synthesized in the presence of and capped by thioglycolic acid, 3-mercaptopropionic acid, mercaptoethanol or a-thioglycerol ligands display zinc blende nanocrystalline structures, CdSe QDs modified by L-cysteine possess wurtzite nanocrystalline structures, because different end groups in these ligands induce distinctive nucleation and growth mechanisms. Carboxyl end groups in the ligand served to increase the SPV response of the QDs, when illuminated by hn  Eg,nano-CdSe. Increased length of the alkyl chains and side-chain radicals in the ligands partially inhibit photo-generated free charge carrier (FCC) transfer transitions of CdSe QDs illuminated by photon energy of 4.13 to 2.14 eV. The terminal hydroxyl group might better accommodate energy released in the non-radiative de-excitation process of photo-generated FCCs in the ligand's lowest unoccupied molecular orbital in the 300e580 nm wavelength region, when compared with other ligand end groups. © 2014 Elsevier B.V. All rights reserved.

Keywords: Semiconductors Chemical synthesis Photoelectron spectroscopy Transport properties

1. Introduction

* Corresponding author. Tel.: þ86 335 8074631; fax: þ86 335 8057047. E-mail address: [email protected] (K. Li). http://dx.doi.org/10.1016/j.matchemphys.2014.07.042 0254-0584/© 2014 Elsevier B.V. All rights reserved.

Semiconductor quantum dots (QDs) have great potential for a wide range of applications, such as next-generation biomedicines [1,2], solar cells [3e5] and photoelectron devices [6e8] because of

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their unique electrical, optical, and magnetic properties. Over the last decade, the development of ligand-stabilized CdSe QDs, a typical example of particularly high-quality QDs consisting of II/VI semiconductors, have stimulated research interests in many fields [9,10]. These studies used fluorescence and absorption measurements to mainly focus on the quantum confinement effect, quantum yield, minority carrier lifetime and exciton pair excitation-complex mechanism of the QDs [1,11e13]. Methods for CdSe QD synthesis include organometallic [14e16] and waterphase [17e19] processes. In the present study, water-soluble CdSe QDs were synthesized with the latter approach, because the water-phase synthesis is a simple and low-cost method, and the synthesized QDs can be directly applied in biomedical fields [20,21]. To date, a range of ligands have been used to prepare the QDs, including polyethylene glycol, bovine serum albumin,1 pyridine [22,23], oleic acid [24], sulfhydryl compounds such as thioglycolic acid (TGA) and L-cysteine (L-Cys) [12,25]. These ligands serve mainly to avoid surface defects and agglomeration of the nanoparticles [22,26,27]. In recent years, researchers have noted the effects of the adopted ligand on the surface properties and optical and photoelectron characteristics of QDs. Sharma et al. [23] studied the role of different ligands on the transient absorption characteristics of CdSe QDs based on transient absorption spectral results. The adsorption behaviour of both cysteine ligands and CdSe QDs on TiO2 nanoparticles was investigated using attenuated total reflectance-Fourier transform infrared spectroscopy [28]. The effect of surface passivation on the optoelectronic properties of CdSe QDs capped by oleic acid was discussed on the basis of the absorption and photoluminescence emission spectra [29]. In those studies, the surface photovoltaic (SPV) technique was rarely used to probe the photoelectronic characteristics of ligand-capped CdSe QDs, even though it is an efficient tool to interrogate the surface electronic structure and photoexcited free charge carriers' (FCCs') transport behaviour at semiconductor surfaces and grain boundaries [30,31]. As preparatory works of the present study, we probed the charge transport mechanism in CdTe QDs capped by various ligands using a combination of surface photovoltaic and photoacoustic techniques, in which it was experimentally confirmed that a layer of CdTe1xSx (0 < x  1) formed at the interface of the nanocrystalline CdTe particles and the ligand [32]. Transient photovoltaic technology was used to detect the microdynamic behaviour of the photo-generated FCCs in CdTe/ligand selfassembled QDs [33]. Fine band structure of the ligand-capped CdSe QDs was revealed with SPV technology [34]. It is generally believed that photoinduced FCC transport in heterojunction semiconductors that contain a CdSe/CdS interface originate from energy level alignment between the CdS cap layer and CdSe, and the interfacial defect states [23,35]. However, the effects of added ligands on the surface photovoltaic characteristics and photogenerated FCC transport behaviour in CdSe QDs are not yet fully understood. In the present work, the surface photovoltaic characteristics of CdSe QDs capped by various ligands (sulfhydryl compounds containing various functional groups) were investigated according to SPV and electric field-induced SPV (EFISPV) detection results, supplemented by X-ray diffractometry, Fouriertransform-infrared spectroscopy, and ultravioletevisible absorption spectroscopy. We aim to reveal the influence of ligand capping on QDs consisting of II/VI semiconductors with regard to both nanostructure and photo-generated FCC transport behaviour in the QD coreeshell structure. We believe that our results will be useful in the design of quantum devices with specific microstructures, especially the surface electron structure, using a selfassembly process of the QDs' coreeshell structure without employing other additional technologies.

2. Experimental 2.1. Sample preparation CdSe nanocrystals were prepared according to a previously published aqueous synthesis at room temperature [36]. Briefly, freshly prepared NaHSe solution was injected into a solution of CdCl2 and ligand (TGA, 3-mercaptopropionic acid (MPA), L-Cys, mercaptoethanol (ME), or a-thioglycerol (TG)), which deoxygenated with N2 for 30 min at pH 6.2. The concentration of CdCl2 was 0.1 M and the Cd2þ:Se2:ligand molar ratio was 1:2.4:0.2. The crude solution was refluxed at 100  C for 10 h to obtain CdSe nanocrystals at pH 11. In the present paper, the prepared samples are denoted as CdSe/TGA, CdSe/MPA, CdSe/L-Cys, CdSe/ME, and CdSe/TG, which represent CdSe QDs obtained using the TGA, MPA, L-Cys, ME, and TG ligands, respectively. 2.2. Characterization The average particle size and phase of the samples were measured by X-ray diffractometry (XRD, Rigaku D/max-2500/PC, Japan), and the data obtained were analysed using the MDI Jade 6.0 software. High-resolution transmission electron micrographs (HRTEM) and selected-area electron diffraction (SAED) patterns were recorded on a JEOL-2010 electron microscope (Tokyo, Japan) operating at 200 kV. Fourier-transform-infrared (FT-IR) spectroscopy (EQUINOX55, Bruker, Germany) was used to determine the functional groups present in the specimens. Ultravioletevisible (UVeVIS) optical absorption spectra of the samples in aqueous solutions were acquired on a Shimadzu UV-2550 spectrophotometer (Kyoto, Japan). All optical measurements were performed at room temperature. Scheme 1 depicts a self-assembly surface photovoltaic detection set-up. The principle and experimental details of SPV and EFISPV spectroscopies have been described elsewhere [30,31]. The SPV methods can be used effectively to obtain information about photogenerated charge transfer (CT) transition behaviour and electronic structure at surfaces and phase interfaces, even in block materials except for detecting SPV characteristics of semiconductors, because the techniques are by no means sensitive only to surfaces. Rather, they are sensitive to the entire surface space charge regions by super- or sub-band-gap absorption, even to buried interfaces located anywhere in the sample, as long as they can be reached by photons [30,37,38]. EFISPV spectroscopy can provide further details about the directions of the built-in field at semiconductor surface space charge region and the diffusion of photo-generated carriers.

Scheme 1. Diagrammatic sketch of the experimental setup for surface photovoltaic spectroscopy, in which the irradiation is a 500-W Xe-arc lamp, the white thick line highlights the non-modulated light, the coloured thick lines monochromatic lights of ultraviolet to near infrared, the coloured dashed lines the modulated light, and the thin solid lines the cable. The SPV measurement was carried out under room temperature.

K. Li, Z. Xue / Materials Chemistry and Physics 148 (2014) 253e261

3. Results and discussion 3.1. Microstructural characteristics

Table 1 Diffraction angle, lattice, and grain size of the four samples with zinc blende structure. Sample

Fig. 1 shows XRD patterns of the prepared CdSe QDs adopting different ligands. We begin with a discussion of the XRD patterns of the CdSe/TG, CdSe/ME, CdSe/MPA and CdSe/TGA samples. Three XRD peaks are seen in each of the four samples (labelled (b), (c), (d) and (e) in Fig. 1) and correspond to the (111), (220) and (311) planes of the cubic zinc blende CdSe system, according to standard XRD spectra PDF#19-0191 from the IDCC database. Compared with the standard spectra of CdSe crystal, the prepared CdSe samples have broadened peaks, which may result from the small synthesised particle sizes, as listed in Table 1. It is interesting that the (220) peak of the four samples in Fig. 1 showed an obvious shift to the standard spectrum of the CdS crystal. This implies that during the synthesis, the sulfhydryl group in the ligands such as TGA, MPA, Me and TG coordinate more easily with Cd2þ ion vacancies on the (220) plane than on two other lattice planes, then form a CdS layer between the core-CdSe nanoparticle and the particular ligand. According to Ref. [12], this may be related to the fact that the (220) plane results from thermodynamically-preferred grain growth, while the (111) plane forms from a random nucleation mechanism. Consequently, the formation of the self-assembled CdS layer was mainly dominated by the (220) plane that can improve the CdSe QDs electronic properties like most semiconductor devices [39], confirming that a CdS shell layer was formed between the CdSe nanoparticles and their ligands, as opposed to a CdSeS alloy. In addition, it was noticed that the intensity of (111) peak of the CdSe/TG and CdSe/ME samples are noticeably higher than that of CdSe/MPA and CdSe/TGA. This indicates that the nucleation and growth of the coreeshell CdSe QD structure was influenced by the space effect of certain ligand groups. Specifically, hydroxyl groups in the ME and TG ligands may be more able to induce random nucleation of CdSe along the (111) plane rather than to encourage growth of the (220) and (311) planes in comparison with carboxyl groups in the MPA and TGA ligands. In contrast to the four samples described above, the XRD spectrum of the CdSe/L-Cys sample displayed a hexagonal wurtzite structure with (002), (102), (110), (103) and (200) peaks located at 25.456, 35.686, 42.285, 45.709 and 49.859 , respectively, as seen in curve (a) in Fig. 1. Formation of the wurtzite structure can probably be attributed to complex interactions of the amino and carboxyl

Fig. 1. XRD patterns of the samples CdSe/L-Cys (a), CdSe/TG (b), CdSe/ME (c), CdSe/ MPA (d), and CdSe/TGA (e).

255

Cubic CdSe Cubic CdS CdSe/TGA CdSe/MPA CdSe/ME CdSe/TG

(111)

(220)

(311)

DxRD (nm)

2q ( )

d (nm)

2q (  )

d (nm)

2q (  )

d (nm)

25.354 26.506 25.109 25.229 25.145 25.694

3.510 3.360 3.544 3.527 3.539 3.514

42.008 43.960 42.551 42.989 42.754 43.001

2.149 2.058 2.136 2.117 2.119 2.102

49.698 52.132 49.878 49.889 49.395 49.787

1.833 1.753 1.827 1.826 1.843 1.830

e e 2.00 1.93 1.81 1.66

groups in the L-Cys ligand on the crystalline phase of the CdSe/LCys. In addition, the (002) plane did not shift to higher angles, unlike the (110) and (200) planes. This implies that the latter two are more liable to form a CdS shell layer between the hexagonal CdSe nanoparticles and the ligand L-Cys than the former. TEM images of the CdSe/TGA and CdSe/L-Cys samples are shown in Fig. 2a and d, in which the average grain size of the two samples was determined to be 2.14 nm and 2.26 nm, respectively. Clear lattice fringes were spaced at about 0.35 nm, as revealed in the HRTEM images seen in Fig. 2b and e. The SAED patterns of the two samples in Fig. 2d and f confirmed that the CdSe/TGA sample belonged to the cubic system with (111), (220) and (311) planes, while the CdSe/L-Cys sample displayed hexagonal structure with (002), (102), (110), (103) and (112) planes. These findings were in a good agreement with the XRD results of the two samples in Fig. 1. The bright rings in the SAED patterns indicated that the samples prepared were similar to polycrystalline cadmium selenide. To confirm the microstructure of the prepared CdSe nanoparticles capped by various ligands, Fig. 3 displays FT-IR spectra for each of the five samples. Characteristic IR absorption bands of the pure ligands (TGA, MPA, L-Cys, ME and TG; dotted lines in Fig. 3) and the samples (CdSe/TGA, CdSe/MPA, CdSe/L-Cys, CdSe/ME and CdSe/TG; solid lines in Fig. 3) are listed in Table 2. The nOeH (COOH), nSeH and dOeH (COOH) absorption peaks were specific to the TGA, MPA and L-Cys ligands, but disappeared in the IR absorption spectra of the corresponding samples as seen when comparing the dotted and solid lines in Fig. 3a, b and c, respectively. The nCH2, dCH2 and nC]O (COOH) absorption bands appeared simultaneously in the IR spectra of the ligands and the samples in Fig. 3a, b or c, except for s exhibiting the vibration bands nas COO and nCOO. The only thing that was different about those vibration bands was the appearance of an IR sum-frequency absorption peak for the L-Cys ligand at 2082 cm1 (dotted line in Fig. 3c), owing to a deformation of the NH2 asymmetric vibration resulting from the eCOOH vibration. However, this peak disappeared in the QD spectrum (solid line seen Fig. 3c). The nCH2, dCH2 and nCeO (in alcohol) peaks occurred in the IR spectra of the CdSe/ME and CdSe/TG samples, although the IR spectra of the ME and TG ligands also exhibited nOeH (in alcohol), nSeH and dOeH (in alcohol) vibrational peaks, except for the three vibration peaks mentioned above, as seen in Fig. 3d and e. It is important to note that the nSeH peak present for all five ligands was absent in the IR spectra of all five CdSe QD samples in Fig. 3aee. At the same time, the nCeS peak appeared in the IR spectra of the five CdSe samples. All IR absorption characteristics implied that the S atom in the eHS group of the ligands had partially replaced a Se atom in CdSe, and then formed a CdS layer on the CdSe nanoparticle surface. The absorption bands located respectively at 1065 cm1 and 1043 cm1 in the dotted and solid lines in Fig. 3c are ascribed to the characteristic CeN stretching vibration, according to Ref. [28]. These assignments are consistent with the XRD and SAED results for the CdSe/L-Cys sample in Figs. 1 and 2.

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Fig. 2. TEM and HRTEM images, and SAED patterns of the samples CdSe/TGA (a, b and c) and CdSe/L-Cys (d, e and f).

Table 2 also lists the electronegativities of relevant elements and groups according to Refs. [40,41]. As is well known, the smaller the electronegativity difference (namely, the lower the charge density difference) between two microparticles, the easier covalent bond formation between them in the nucleation and growth process. According to the data listed in Table 2, the D-value between Cd and Se is the smallest among those microparticles, and the D-value between Cd and S (or a sulfhydryl group) is only slightly higher. However, the largest D-value arises between the carboxyl (or hydroxyl) group and Cd. This might result in an even greater difficulty to form a covalent bond for the latter as compared with the first two. Consequently, a schematic diagram of the coreeshell structure of the ligand-capped CdSe nanoparticles is illustrated at the top of Fig. 3a, in which the letter R represents the respective end groups in the adopted ligands as shown in Fig. 3aee, respectively. In addition, the D-values above could also cause steric and electronic inductive effects, directly influencing the CdSe QD growth mechanism. For example, the effect of having hydroxyl groups in the capping agent, such as TGA, on nucleation and growth of the QDs might contribute to enhanced growth at the (220) plane rather than the (111) and (311) planes in a zinc blende structure. This was probably responsible for the obvious cubic structure of the CdSe/TGA sample. The LCys ligand was more effective at inducing multiple orientation growth of the QDs, resulting in a hexagonal phase structure of the CdSe/L-Cys sample, in contrast with CdSe/TGA, because the large electronegativity of the side-amidogen group in the L-Cys could lead to a strong steric and electronic inductive effect. Consequently, the nanostructure of the CdSe QDs prepared was closely related to the electronic density and space-induction effects of the end groups in the ligand capping agents. 3.2. Photoelectron characteristics 3.2.1. UVeVIS absorption analysis Fig. 4a shows the UVeVIS absorption spectra of the CdSe/TGA, CdSe/MPA, CdSe/L-Cys, CdSe/ME and CdSe/TG samples. The average

grain size DUV-VIS (nm) of the samples was calculated according to Ref. [4]. The calculated results are consistent with that obtained from the XRD patterns and TEM images, as listed in Table 3. By comparing the diameters, DUVeVIS, of the five samples, it was noticed that ligands that only contained hydroxyl groups rather than carboxyl groups, such as ME and TG, led to smaller DUVeVIS values in their corresponding CdSe nanoparticle samples than in others. According to Refs. [42,43], increased separation speed of the precursor Cd-ligand can accelerate grain growth because it provides many more active sites on the precursor. If the ligand was highly water soluble, it would contribute to precursor formation, and would speed up its decomposition process. Therefore, we suppose that the lower water solubility of the CdSe/ME and CdSe/ TG samples may be responsible for them having smaller grain sizes than that of the other three samples. Because the average grain sizes of the prepared CdSe QDs were much smaller than the excitonic Bohr radius of CdSe, aB ¼ 5.17 nm, the optical transition energy of the samples can be estimated by the so-called nanocrystal strong confinement effect [44]. Here, the lowest excited state energy (LESE) is obtained by

Eg ¼ E0 þ

Z2 p2 2m D2

1 1 1 ¼ þ m m*e m*h

(1)

(2)

where Eg is the LESE of the nanocrystalline particles; D is the grain size obtained from the UVeVIS spectra; and E0 is the band gap of bulk CdSe at 300 K (E0 ¼ 1.675 eV). The effective masses of electrons and holes of CdSe, m*e and m*h , are 0.119m0 and 0.57m0, respectively [45]. Using Equations (1) and (2), the LESEs of the samples, EgUVeVIS, were obtained and are listed in Table 3. We will use these results to aid our subsequent discussion of the electron structure and the photo-generation FCCs' transport behaviours in the CdSe QDs capped by various ligands according to their SPV

K. Li, Z. Xue / Materials Chemistry and Physics 148 (2014) 253e261

Fig. 3. FI-IR spectra of the pure ligands (dot curves) and the ligand-capped CdSe QDs (solid curves). The microstructural model of ligand-capped CdSe nanoparticles with a coreeshell structure is put at the top in the FI-IR spectra (a); The end group chains opposite to sulfhydryl group in the ligands TGA, MPA, L-Cys, ME, and TG, R(ligand), are shown on the bottom of the FI-IR spectra (a), (b), (c), (d), and (e), respectively.

characteristics. In addition, the two equations were adopted here because the calculated results were largely consistent with our results obtained with the SPV method mentioned below, even though some literature has suggested inconsistency with their experimental results [46]. It was also noted that other absorption peaks appeared at 359 nm, except for a peak related to the mainband gap at 488 nm, as indicated by the arrows in Fig. 4a. 3.2.2. SPV and EFISPV spectroscopy analysis As a typical example, Fig. 4b displays the SPV spectra for the CdSe/L-Cys and Cd/L-Cys samples. The synthesis process of the two

257

samples was similar except that the latter did not include a step for injection of the Se precursor into CdCl2 solution. A probable microstructural model of the Cd/L-Cys is shown in the top-right corner in Fig. 4b. First of all, the positive SPV response of the samples exhibited n-type SPV characteristics, according to Ref. [47]. The peaks marked “1” and “2” simultaneously appeared in the SPV spectra of the two samples, located at wavelengths of 364 nm and 400 nm of Fig. 4b, respectively. Their coincident appearance implied that the two peaks may be respectively related to two bandeband transitions of the outer coordination ligand L-Cys and the shell-CdS (or CdS nanoparticles). Fig. 5 illustrates a schematic diagram of the energy bands corresponding to the photo-generated FCC transport of the samples, based on SPV and UVeVIS results for the prepared CdSe QDs. Peaks 1 and 2 in Fig. 4b are related closely to the transitions labelled by arrows T1 and T2 in Fig. 5, respectively. Furthermore, according to the abscissa of the largest external tangent of the band, the calculated photoelectric threshold of peak 1 was about 3.35 eV, which corresponds to the band gap of the ligand, Eg, ligand, in Fig. 5, while the threshold energy of peak 2 was 2.76 eV, which is equivalent to the band gap of the CdS shell layer, Eg, shell-CdS, in Fig. 5 [48]. In the same way, the threshold energy of peak 3 in Fig. 4b was 2.42 eV. This value can be identified as the band gap of the core-CdSe nanoparticles, Eg, core-CdSe, in Fig. 5, because it is basically equal to the optical band gap of the CdSe/LCys sample, Eg-UV-VIS ¼ 2.46 eV, as listed in Table 3. If the photonic energy (hn1, hn2, or hn3) was greater than or equal to the threshold energies (2.42 eV, 2.76 eV or 3.35 eV, respectively), the CT transitions represented by arrows T1, T2 or T3 in Fig. 5 would induce their respective photo-generated FCC transport processes. Free conduction band electrons produced by the T1 transition move into the bulk from the surface of the core-CdSe nanoparticles in the opposite direction of holes in the valence band, as indicated by the arrows M1 and M2 in Fig. 5. This should result in a reduction of the surface barrier of the core-CdSe from DVs1 to DVs2 in Fig. 5, causing the positive SPV response in Fig. 4b. In addition, the threshold energy of peak 4 in Fig. 4b is smaller than Eg, core-CdSe according to the definition above. Therefore, we suppose that the CT transition related to peak 4 might result from sub-band-gap CT transitions between the energy band and surface states located at Et, 2 in Fig. 5. SPV spectra of the CdSe/TGA, CdSe/MPA, CdSe/L-Cys, CdSe/ME and CdSe/TG samples are shown in Fig. 4c. The LESE (Eg-UVeVIS) and the photoelectric thresholds (Eg-SPS) of the core-CdSe nanoparticles in the five samples are listed in Table 3 according to their respective UVeVIS absorption and SPV spectral results, along with the average grain sizes (DxRD, DTEM and DUVeVIS) of the prepared samples obtained by the XRD, TEM and UVeVIS methods. Comparison of the data in Table 3 reveals that the band gap of the core-CdSe nanoparticles increased as the average grain size decreased because of the quantum size effect. It is important to note that the effect stemmed from the choice of ligands in our experiment. More specifically, the SPV intensity in the 300e580 nm wavelength region for the CdSe/TGA and CdSe/MPA samples, which adopted a growth mechanism resulting in thermodynamically preferred grains at the (220) face of zinc blende nanocrystals, was stronger than that of the CdSe/L-Cys, which has a wurtzite nanostructure. Moreover, the SPV intensity of CdSe/TGA and CdSe/MPA samples, which are capped by modifiers with carboxyl end groups, in the 300e580 nm region was much higher than that of CdSe/TG and CdSe/ME, even though they both assumed zinc blende nanocrystalline structures. It is interesting that, among the SPV response peaks appearing in the region of 300e580 nm (see Fig. 4c), the strongest SPV peak response of any of the CdSe/TGA, CdSe/MPA and CdSe/L-Cys samples was located at a wavelength corresponding to the bandeband transition of the core-CdSe. The SPV intensity at these wavelengths gradually decreased as the length of the alkyl

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Table 2 Vibration band in the FT-IR spectra of the pure ligands and the ligand-capped CdSe QDs (cm1). And electronegativity of the related elements and groups. Vibration band

nOeH dOeH nC]O nas COO ns COO nCeO nCH2 dCH2 nSeH nCeS

Sample TGA

CdSe/TGA

MPA

CdSe/MPA

L-Cys

CdSe/L-Cys

ME

CdSe/ME

TG

CdSe/TG

3090 1155 1714 e e e 2930 1418 2572 670

3362 e e 1571 1388 e 2903 1388 e 684

3083 1159 1720 e e e 2931 1407 2568 670

3398 e e 1563 1401 e 2951 1401 e 670

3055 1144 1600 e e e 2986 1423 2557 635

3416 e e 1588 1401 e 2961 1401 e 677

3367 e e e

3354 e e e

3367 e e e

3401 e e e

1048 2936 1418 2557 633

1000 2939 1418 e 656

1034 2933 1417 2557 629

1074 2923 1423 e 622

Elements or groups

Cd

Se

S

eSH

eNH2

eOH

eCOOH

Electronegativity

1.69

2.55

2.58

2.77

2.78

3.08

3.12

chain and side-chain radical in the relevant ligands was increased from CdSe/TGA to CdSe/MPA and to CdSe/L-Cys. This implied that the carboxyl end group functionality in the ligand might contribute to the SPV effect over the entire SPV response region of the QDs. On the contrary, a long alkyl chain and side-chain radical in the ligand might play an inhibitory role on the SPV characteristics of the CdSe QDs. However, when compared with samples capped by carboxylterminated modifier molecules, nanoparticles capped by hydroxylterminated modifier molecules, such as CdSe/TG and CdSe/ME, the SPV response corresponding to the bandeband transition of the core-CdSe displayed an obvious blue shift and depressed intensity, as seen in Fig. 4c. The first case might be attributable to quantum confinement and size effects of the QDs. This may be false, however, if the depressed SPV intensity of the two samples was ascribed to these quantum effects [23,49]. Therefore, we also have to consider the role the adopted ligand plays in the photo-generated FCCs' transport processes of the QDs. Combining the data in Table 3 with the QD energy band diagram in Fig. 5, the free electron and hole caused by the T1 transition may also diffuse into the shell-CdS state that looks like a trap, if they existed as an exciton pair as indicated by the hollow arrows D1 and D2 in Fig. 5. This case should lead to a faster degradation of the SPV response signals resulting from the bandeband transition of the core-CdSe located at a specific wavelength, when compared with

Fig. 4. (a) UVeVIS absorbance spectra of the samples CdSe/L-Cys, CdSe/TG, CdSe/ME, CdSe/MPA, and CdSe/TGA, in which the two arrows are explained in detail in the text. (b) Surface photovoltaic spectra of the samples CdSe/L-Cys and Cd/L-Cys. The microstructural model of the sample Cd/L-Cys (Rligand is eCH2eCH(NH2)eCOONa) is shown at the top-right corner. (c) Surface photovoltaic spectra of the samples CdSe/L-Cys, CdSe/TG, CdSe/ME, CdSe/MPA, and CdSe/TGA.

the SPV intensity of the shell-CdS at 398 nm as mentioned above, as is seen in the SPV spectra of the CdSe/TGA, CdSe/MPA and CdSe/LCys samples shown in Fig. 4c. As an exciton pair, the free electron and hole, which are produced by the T3 transition from the ligand highest occupied molecular orbital (HOMO) to its lowest unoccupied molecular orbital (LUMO), might move together into the surface space charge region of the core-CdSe by quantum tunnelling as indicated by the hollow arrows D3 and D4 in Fig. 5, because the average grain sizes of the samples were much smaller than the excitonic Bohr radius of CdSe. Then, the exciton pair species could move respectively to the bulk and the surface of the core-CdSe as the arrows M3 and M4 illustrate in Fig. 5. The quantum tunnelling Table 3 Average grain size of the CdSe nanoparticles prepared and band-gap of the coreCdSe nanoparticles. Sample

DxRD (nm)a DTEM (nm)a DUVeVIS (nm)a Eg-UVeVIS (eV)b Eg-SPS (eV)b

CdSe/L-Cys CdSe/TGA CdSe/MPA CdSe/ME CdSe/TG

2.09 2.00 1.93 1.81 1.66

2.26 2.14 2.10 1.88 1.77

2.21 2.14 2.10 1.95 1.80

2.46 2.51 2.54 2.68 2.85

2.42 2.50 2.51 2.61 2.89

a Average particle sizes DxRD, DTEM, and DUVeVIS of the prepared CdSe QDs were calculated by the XRD, TEM, and UVeVIS adsorption spectra, respectively. b Band-gap Eg, core-CdSe of the prepared CdSe QDs was derived from the UVeVIS adsorption spectra (be referred to Eg-UVeVIS) and the SPV spectroscopes (be referred to Eg-SPS), respectively.

Fig. 5. Schematic diagram of the energy levels and probably transport mechanism of photo-generated FCCs in the ligand-capped CdSe QDs with a coreeshell structure, in which the arrows and the symbols are explained in detail in the text. Here the described interface potential barrier between the core-CdSe and the shell-CdS was based on an obvious n-type SPV characteristic of the samples as discussed in the text. And the thickness of the shell-CdS was about a few molecules according to the results of XRD and FI-IR spectra of the five samples.

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effect may enlarge the diffusion length of the photo-generation FCCs in the CdSe QDs, leading to further reduction of the barrier DVs2, while simultaneously increasing the core-CdSe SPV response. The low SPV response of the CdSe/L-Cys (see Fig. 4c), as illuminated by the photonic energy hn1  Eg, core-CdSe, may result from an inconspicuous quantum tunnelling mechanism, when compared with the CdSe/TGA and CdSe/MPA samples. According to previous literature [50], the outer-layer ligand can interrupt the carriers' transport between heterojunctions through non-radiative energy transfer to the vibrational overtones of some chains and groups of the ligand. Therefore, we can further infer that the gradually decreasing SPV response in the 300e580 nm region in Fig. 4c, when going from CdSe/TGA to CdSe/MPA to CdSe/L-Cys, could be explained by the MPA molecule having one more CH2 group than the TGA molecule, and the L-Cys molecule having an extra NH2 group. The excess end groups in the ligands might accommodate some higher-energy oscillations caused by the non-radiative transition, N1, from the LUMO level to the energy level Et1 in the ligand energyegap, reducing the extent of the exciton pair's migration, D3 and D4, that were caused by quantum tunnelling, and lowering the degree of reduction to the surface barrier DVs2 in Fig. 5. This could finally lead to the gradual SPV intensity degradation seen in the series of CdSe/TGA, CdSe/MPA and CdSe/L-Cys samples in Fig. 4c. For the same reason, the extra OH end group in the TG molecule, when compared with the ME molecule, should be responsible for CdSe/TG having a lower SPV intensity than CdSe/ME, as seen in Fig. 4c. More specifically, the OH end group of the TG ligand might accommodate energy released by the non-radiative transition N1 in Fig. 5, and become a higher energy oscillator just as the situations mentioned above. Consequently, CdSe/TG showed the weakest SPV response among all five samples, because the extra functional groups in the TG molecule cap on the CdSe QDs might allow a more

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obvious non-radiative transition process from the LUMO to the OH group electron levels in the ligand than in the others. To confirm our hypotheses above, in Fig. 6aee we plot the SPV intensity as a function of changing external electric field at the specific wavelength of the main-band-gap transition of the coreCdSe for each of the five samples. There were two discrepancies seen among the electric field-induced surface photovoltaic (EFISPV, see Supporting information) response intensity trends for these samples. Unlike the other four samples in Fig. 6, the SPV response intensity of CdSe/L-Cys increased with increasing external positive electric field from 0 V to 5 V, but then decreased as the absolute value of the external negative electric field increased from 0 V to 5 V at the wavelength of 469 nm (Fig. 6a). This EFISPV behaviour of the CdSe/L-Cys may be attributed to its wurtzite nanocrystalline structure, which demonstrates more obvious n-type SPV characteristics as compared with the other four samples because the direction of the external positive electric field was coincident with the internal field of the QDs, contrary to external negative field conditions. This property of the CdSe/L-Cys should result in a consistent moving direction of the photo-generated FCCs with the same charge in the whole QD coreeshell structure. This means that quantum tunnelling may seldom appear in FCC transport processes of the CdSe QDs modified by LL-Cysteine. Another delineation can be seen among the other four samples, even though they all assume a zinc blende structure. Specifically, the SPV intensity of CdSe/TGA and CdSe/MPA decreased as the magnitude of the external field was increased e whether positive or negative e at their respective specific wavelength shown in Fig. 6b and c, respectively. However, the EFISPV response of CdSe/TG and CdSe/ME initially increased with increasing magnitude of the applied field, but then decreased upon further field increases (in both the negative and positive directions), as shown in Fig. 6d and e. This implies that the surface

Fig. 6. Changes of the SPV response intensity of the samples CdSe/L-Cys (a), CdSe/TGA (b), CdSe/MPA (c), CdSe/ME (d), and CdSe/TG (e) at the specific wavelength related closely to the main-band-gap transition of the core-CdSe as a function of external electric field, respectively.

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electron structure and the FCC transport behaviour for the first two systems are different from the latter two. The EFISPV characteristics of the CdSe/TGA and CdSe/MPA samples may stem from quantum tunnelling in the QD coreeshell structure because the photogenerated FCCs responsible for quantum tunnelling move in a direction opposite to that of the internal field of the QDs, resulting in a SPV intensity decrease as the external positive field increases, as shown in Fig. 6b and c. On the contrary, unobvious quantum tunnelling of the CdSe QDs modified by the mercaptoethanol or athioglycerol molecules was unable to guide more photo-generated FCCs across it, causing the SPV intensity to initially increase, but then decrease as the absolute value of the external field increased, as seen in Fig. 6d and e. This behaviour is in good agreement with the observed UVeVIS and SPV results in Fig. 4aec. Obviously, the functional groups in the ligand molecules of the five samples should be responsible for their respective EFISPV characteristics above. 4. Conclusions In summary, it was confirmed that the nanostructure and the photo-generated FCCs' transport characteristics of CdSe QDs may be regulated by a self-assembly process of the QD coreeshell structure without further additional technology, according to SPV and EFISPV results, supplemented by X-ray diffractometry, infrared absorption spectroscopy, and UVeVIS absorption spectroscopy. First, we found that the nanostructure of the prepared CdSe QDs was closely related to the electronic density and space-induction effect of the end groups on the QD ligand caps. For example, the CdSe QDs prepared by ligand capping with TGA, MPA, ME, or TG had a zinc blende nanocrystalline structure. More specifically, while hydroxyl groups in the ME and TG ligands induced random nucleation of CdSe along the (111) plane, carboxyl groups in the MPA and TGA ligands encouraged growth of the (220) face that may dominate the QD electric properties and give preference for formation of a CdS shell layer between the core-CdSe and the outer layer ligand. However, the complex impact of amidogen and carboxyl groups in the L-Cys ligand on the nanocrystalline phase of the CdSe QDs resulted in a wurtzite nanocrystalline structure, in which the (110) and (200) planes were more suitable for forming the CdS shell layer than the other three planes. Second, CdSe QDs capped by the TGA and MPA ligands possessed more obvious quantum tunnelling than those capped by L-Cys because the nucleation and growth mechanisms of the first two were distinct from the latter, even though all three ligands contain a carboxyl end group. This resulted in a strong SPV response in the CdSe QDs capped by the TGA and MPA ligands, especially when illuminated by the photonic energy hn  2.42 eV, as compared with that capped by L-Cys. In addition, the heterostructure of the coreeshell system that first formed at the (110) and (200) planes in the wurtzite CdSe nanocrystals was partially responsible for the CdSe/L-Cys QDs having the lowest SPV intensity among these three samples in the 300e580 nm wavelength region. Finally, the hydroxyl end group of the ME and TG ligands might better accommodate energy released in the non-radiative de-excitation process of photo-generated FCCs in the LUMO of the ligand, compared with other ligand end groups. This effect might also reduce quantum tunnelling in the coreeshell system and weaken the SPV characteristics of the CdSe QDs. Moreover, increased length of the alkyl chain and side-chain radical in the ligand was partially responsible for the decreased surface photovoltaic effect of the CdSe QDs when illuminated at photonic energies greater than or equal to the photoelectric threshold of the core-CdSe nanoparticles, because they might play a non-negligible role in reducing the diffusion length of the photo-generated FCCs in the QDs.

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