1,2,3-Selenadiazole-driven single family MSNCs of CdSe

Article abstract of DOI:10.1039/c7nj02994b

Herein, 1,2,3-selenadiazoles were instantly prepared by hand grinding the reactants via a solventless process and tested for their ability to act as a source of selenium for the synthesis of CdSe magic-sized nanoclusters (MSNCs) with size below 2 nm. The

Full text of DOI:10.1039/c7nj02994b

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1,2,3-Selenadiazole-driven single family MSNCs
of CdSe†
Cite this:DOI: 10.1039/c7nj02994b
Aditi A. Jadhav and Pawan K. Khanna
*
Herein, 1,2,3-selenadiazoles were instantly prepared by hand grinding the reactants via a solventless
process and tested for their ability to act as a source of selenium for the synthesis of CdSe magic-sized
nanoclusters (MSNCs) with size below
2 nm. The current study describes the use of different
1,2,3-selenadiazole derivatives, viz. small chain aliphatic, long chain aliphatic, acetophenone, and
aromatic derivatives, in the preparation of CdSe MSNCs. The study was carried out to check the effects
of different functionalities of the precursor on the formation of MSNCs of CdSe. The MSNCs prepared
Received 12th August 2017,
Accepted 23rd October 2017
using
a variety of selenium precursors were analyzed by various spectroscopic techniques.
A characteristic doublet in the absorption spectra and broad emission profile consistently indicated that
the employed selenadiazoles were useful for the selective fabrication of single family MSNCs of CdSe,
and the formation of regular quantum dots was suppressed.
DOI: 10.1039/c7nj02994b
rsc.li/njc
The growth occurs in a definite manner to acquire a sym-
metrical stable cluster structure. Further stepwise growth takes
1. Introduction
Clusters are aggregates of atoms and molecules with size inter- place by adding a blanket of atoms to the rigid core. Close-
mediate between that of the bulk matter and the individual packed arrangements result in the formation of symmetrical
atoms and molecules. Clusters can be homogeneous or hetero- clusters. The layered and controlled growth of atoms takes
geneous in nature based on their composition driven by either place during the process that leads to the higher stability and
the same type of atoms or different types of atoms. The stability symmetry of the clusters.
of a cluster will depend upon the number of atoms in it. Magic
During the synthesis of magic-sized nanoclusters (MSNCs),
number nanoclusters are also referred to as early stage or it is a common observation that after the well development of
magic-sized nanoclusters (MSNCs) and are normally thermo- the magic clusters, they retain their size and do not grow
dynamically stabilized. The magic number generally refers to further at higher temperatures or after a longer reaction time.
the number of atoms coordinating the central atom in an Wang et al.5 showed that during the crystal nucleation process,
arrangement such that a stable close packed structure generally the nucleation barrier must be less than the chemical reaction
in the size range of 2 nm and below is generated.1 It has been barrier for generating monomers. Accordingly, if a higher
reported that these small clusters are thermodynamically more activation energy is required for the formation of crystals,
stable than their respective larger clusters due to the perfect aggregates of monomers may be formed in the chemical
coordination between their surface and their intrinsic energy.2 reaction instead of crystals.
These tiny nanoclusters are often observed with rice-grain type
As is well known, II–VI semiconductor nanoclusters are of
morphologies based on XRD and TEM analysis. It has been great importance for both fundamental research as well as
further reported that CdSe NCs smaller than 2 nm undergo technical applications.6 In particular, CdSe nanoparticles have
quantized growth, i.e. their size or molecular weight increases been greatly studied in electronics and photonics because of
stepwise in a discrete manner.3 During the initial step of cluster their narrow absorptions and widespread emission profile
formation, the atoms (monomers) reorganize themselves and nearly covering the entire visible spectrum as well as their great
form a morphologically completely different structure. On these potential in white light devices.7–9 However, it is realized that
small clusters, atoms are added at each stage of growth.4 nanoclusters with sizes less than 2 nm have not yet been used
in real applications despite their potential mainly because of
Nanochemistry/QDsR& D Laboratory, Department of Applied Chemistry, Defence
the issues related to their synthesis.
Institute of Advanced Technology (DIAT), Ministry of Defence, DRDO, Govt. of India,
Amongst semiconductor materials, a variety of binary
Girinagar, Pune-411 025, India. E-mail: pawankhanna2002@yahoo.co.in;
MSNCs have been reported including PbSe,10 ZnSe,11 CdS,12
Fax: +020-24389360; Tel: +020-24389360
CdTe,13 and CdSe14 magic clusters. CdSe MSNCs with different
† Electronic supplementary information (ESI) available: Containing analytical
data, Fig. S1–S18. See DOI: 10.1039/c7nj02994b
sizes and shapes have been synthesized, and their properties
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have been studied by many research groups.1,6,7,14–16 White-light 1,2,3-Selenadiazoles are traditional N-heterocycles containing
nanocrystals show an emission pattern, which is more or less selenium, and the reaction of semicarbazones with selenium
identical to sunlight; therefore, they are suitable materials for use dioxide is normally responsible for their chemical synthesis
in solid-state lighting devices. Recently, white-light nanocrystals in via a ring closure mechanism. Almost all the transformations
a polymer matrix have been utilized in prototype photolumines- have been reported using some organic or aqueous solvents.
cent devices.16 The utility of white-light CdSe nanoclusters has We have recently reported a modified potentially greener synthesis
also been investigated for the fabrication of electroluminescent of 1,2,3-selenadiazoles without using any liquid medium.19 In the
devices.17 In addition, magic-sized nanoclusters are useful mate- current study, we have extended our efforts to synthesize several
rials for the construction of biological probes.6,18 Because of their novel 1,2,3-selenadiazoles along with a few previously reported
extremely unique optical properties, these nanoclusters can be examples using our solventless approach, thus generating a range
coupled with a probe having a precise size to make them ideal of these selenium-containing diazoles. Therefore, in this study, we
candidates for multi-target labelling studies. Moreover, it is report the synthesis of a variety of 1,2,3-selenadiazoles and their
advantageous that MSNCs show long-term photostability and reactivity towards cadmium metal salts to exclusively synthesize
can emit multiple colours with a single excitation source; these magic-sized nanoclusters of cadmium selenide. Herein, four
attributes make them useful material for tumour detection using different series of 1,2,3-selenadiazoles were synthesized and
fluorescence spectroscopy.
utilized as a selenium source for the first time. The substituents
Various research groups have studied the synthesis of of selenadiazoles are categorized into three different classes:
MSNCs by manipulating the reaction conditions such as reaction small alkyl group, large alkyl group, and aromatic. The effects of
temperature, capping ligands, and reaction medium, and using these different selenium precursors on the formation of MSNCs
various cadmium and selenium sources. However, most of the were studied and discussed. It was observed that acyclic aliphatic
approaches resulted in the formation of multiple families of 1,2,3-selenadiazoles led to the formation of a single family of
MSNCs; thus, only a few studies on the formation of a single MSNCs. On the other hand, aromatic ring-substituted selena-
family of MSNCs have been reported. Ptatschek et al.15b reported diazoles showed the formation of regular-sized quantum dots
multiple families of CdSe MSNCs synthesized at room tempera- (RSQDs) along with MSNCs. More importantly, the present study
ture in early 1998. These clusters exhibited sharp absorption provides efficient experimental methods and approaches to
peaks at 280 nm, 360 nm, and 410 nm with respect to different single-sized nanoparticles desired for various applications.
sizes of the clusters. Similarly, multiple families of CdSe MSNCs Herein, the selenadiazole precursors were synthesized using a
with absorptions at 330 nm, 350 nm, 384 nm, 406 nm, 431 nm, simple eco-friendly method, and by manipulating the substituent
and 447 nm were reported by Kudera et al.3b Newton et al.32 on the selenadiazoles, single-sized reproducible NCs with desire-
demonstrated a low temperature synthesis of multiple families able sizes and properties could be obtained.
of CdSe MSNCs using various capping ligands, highlighting the
importance of the cadmium precursor in nanocluster growth.
Yu et al.15c timely addressed the synthesis and characterization
of single-sized CdSe nanocrystals exhibiting a band gap absorp-
2. Experimental
2.1 Materials/general
tion peaking at 463 nm using a non-injection one-pot approach.
Later, they realized the controlled formation of magic-sized CdSe All reagents, analytical grade chemicals, and solvents were
quantum dots along with regular quantum dots (MSQDs and commercially obtained from Sigma-Aldrich Company and
RQDs) by effectively manipulating the reaction parameters.15d
Merck India Ltd. UV-visible spectra (in solution) were obtained
The properties of nanoclusters (NCs) are considered to be at room temperature using an Analytik Jena SPECORD 210
greatly dependent on both the composition and size; thus, a PLUS UV spectrophotometer. The Cary Eclipse Fluorescence
synthetic approach with the ability to control both of these Spectrophotometer G9800A (Agilent Technologies, USA) was
variables is of paramount importance. Thus, understanding used for the PL measurements in solution with an appropriate
and controlling the growth of semiconductor QDs are impor- excitation energy; typically, the samples were excited at 350 nm.
tant steps towards developing materials with well-defined The FTIR spectra were obtained in the range of 4000–400 cmÀ1
optical and physical properties. To obtain NCs with a well- using the FTIR Perkin Elmer Spectrum-Two spectrometer. The
defined size and narrow size distribution, the initial reaction samples were analysed by Raman spectroscopy at room tem-
conditions, such as temperature, precursor concentration, perature between 100 cmÀ1 and 3500 cmÀ1 using an EZRAMAN-
1
precursor type, and capping ligands, need to be carefully N-785-B1S of (Enwave Optronics, US). H NMR (500 MHz) and
selected because these parameters can strongly influence the 13C NMR (125 MHz) spectroscopy was performed in CDCl3
balance between nucleation and growth that impacts the using TMS as an internal standard. NMR spectra were obtained
particle or cluster size, re-dispersibility and size distribution, using a Bruker DRX-300 instrument. Mass spectra were acquired
shape, and overall quality.
via a Mass Analyser @ Advion (USA) NY-14850 (ESI/MS) spectro-
Appropriately, the current research is aimed at tuning the meter using electron spray ionization. Powder X-ray diffraction
synthesis of MSNCs of CdSe with relatively greener concepts patterns were obtained via a Mini Flex Rigaku X-ray diffractometer
using a variety of 1,2,3-selenadiazoles that may play a major using Cu-Ka (l = 1.5406 Å) radiation. Energy-dispersive X-ray
role in the near future as possibly the best source of selenium. analysis (EDAX) was carried out using a Carl Zeiss scanning
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electron microscope. TEM images were obtained using FEI- 21.9 (CH2); 23.3 (CH2); 29.5 (CH2); 38.9 (CH2); 44.7 (CH2); 153.9
TECNAI G2 (Czech Republic). The particle size distribution was (CQC–Se); 162.7 (CQC–N). Found, m/z: 233.1 [M] + H+. C9H17N2Se
studied using NANOPHOX (sympa, Germany). Thermogravi- CHN; found %C 46.50; H 7.0; N 12.10; Cal. %C 46.76; H 6.98;
metric analysis (TGA) was performed from room temperature N 12.12.
to 800 1C at a heating rate of 10 1C minÀ1 under a nitrogen
2.2.4. 4-Nitrophenyl-1,2,3-selenadiazole (6). 4-Nitrophenyl-
atmosphere using a Perkin Elmer STA 6000 (simultaneous 1,2,3-selenadiazole (6) was synthesized from 4-nitroaceto-
Thermal Analyser) instrument. CHN analysis was carried out phenone semicarbazone. Yield 3.09 g (48%), yellow solid,
using a vario MICRO Elementar Analysensysteme (Sr. No. Rf 0.60. UV spectrum (toluene), lmax (nm): 343. IR spectrum,
15142018). The progress of the reactions was monitored by
n
(cmÀ1): 3027–3031 (Ar C–H), 2853–2937 (C–H), 1600
TLC via Merck silica gel 60 F-254 aluminium sheets using (Ar CQC), 1690 (CQC), 1342 (–NO2 str.); 1446 (C–H), 1261
EtOAc as the eluent and visualized under UV light. Purification (C–N). 1H NMR spectrum, d (ppm) J (Hz): 8.12–8.14 (2H, m,
of the organic compounds was carried out via flash column Ar CH); 8.32–78.34 (2H, m, Ar CH); 8.70 (1H, s, selenadiazole
chromatography using a Cheetah Purification system (Agela ring H). 13C NMR spectrum, d (ppm): 123.7; 123.8; 124.58;
Technologies-Mod No. MP200).
126.7; 128.3; 129.3 (Ar Cs); 140.1 (CQC–Se); 141.4 (CQC–N).
Found, m/z: 254 [M + H]+. C8H5N3O2Se.
2.2 General method for the synthesis of 1,2,3-selenadiazoles
2.2.5. 5-Ethyl-4-phenyl-1,2,3-selenadiazole (8). 5-Ethyl-4-
The selenadiazole derivatives (1, 2, 5, 7 & 9) were synthesized phenyl-1,2,3-selenadiazole (8) was synthesized analogously to
using a solventless method as previously reported.19 Herein, the other compounds using butyrophenone semicarbazone.
four new 1,2,3-selenadiazoles (3, 4, 6 & 8), which have not been Yield 2.89 g (50%), yellow viscous liquid, Rf 0.62. UV spectrum
reported earlier, have been synthesized using the same above- (toluene), lmax (nm): 300. IR spectrum, n (cmÀ1): 30230–3032
mentioned general method for the first time, and their charac- (Ar. C–H), 2856–29230 (C–H), 1590 (Ar. CQC), 1668 (CQC),
1
terization data are given hereinafter. Typically, these were 1445 (C–H), 1270 (C–N). H NMR spectrum (CDCl3), d (ppm) J
synthesized by hand grinding the corresponding semicarbazones (Hz): 7.16–7.29 (5H, m, Ar. CH); 2.72–2.76 (3H, m, J = 10.0, CH3);
and SeO2 in a 1 : 1 ratio using a mortar and pestle followed by 2.86–2.90 (2H, m, J = 10.0, CH2). 13C NMR spectrum (CDCl3),
extraction with toluene. The products were purified using flash d (ppm): 29.7 (CH3); 30.0 (CH2); 141.0 (CQC–Se); 150.0
column chromatography. The yield was typically in the range of (CQC–N); 126.1 (Ar Cs); 126.3–128.5 (Ar Cs). Found, m/z: 237,
50–55%.
239 [M + H]+. C10H10N2Se CHN; found; %C 50.45; H 4.33;
2.2.1. 4-Ethyl-1,2,3-selenadiazole (1). 4-Ethyl-1,2,3-selenadiazole N 12.00; Cal. %C 50.64; H 4.25; N 11.81.
(1) from 2-butanone semicarbazone, 4-isobutyl-1,2,3-selenadiazole
(2) from isobutylmethyl semicarbazone, 4-(4-hydroxyphenyl)-1,2,3-
selenadiazole (5) from 4-hydroxyacetophenone semicarbazone,
5-phenyl-1,2,3-selenadiazole (7) from propiophenone semicarba-
zone and 4-phenyl-5-propyl-1,2,3-selenadiazole (9) from the
corresponding valerophenone semicarbazone were synthesized
using seleniuos acid as per a literature procedure.19
2.2.2. 5-Ethyl-4-propyl-1,2,3-selenadiazole (3). 5-Ethyl-4-propyl-
1,2,3-selenadiazole (3) was synthesized from 3-hexanone semi-
carbazone following the general abovementioned procedure. Yield
3.27 g (55%), yellow viscous solid, Rf 0.62. UV spectrum (toluene),
lmax (nm): 298. IR spectrum, n (cmÀ1): 2874–2962 (C–H), 1590
(CQC), 1465 (C–H defr.), 1294 (–C–N str.). 1H NMR spectrum,
d (ppm), J (Hz): 0.99–1.02 (3H, m, J = 5.0, CH3); 1.37–1.40 (2H, m,
J = 10.0, CH2); 1.82–1.87 (3H, m, J = 10.0, CH3); 2.94–2.99 (4H, m,
J = 5.0, CH2). 13C NMR spectrum, d (ppm): 13.7 (CH3); 18.6 (CH3);
19.5 (CH2); 31.1 (CH2); 38.9 (CH2); 153.9 (CQC–Se); 158.0
(CQC–N). Found, m/z = 205.09 [M] + H+. C7H13N2Se. CHN; found
%C 41.38; H 6.01; N 13.82. Cal. %C 41.39; H 5.95; N 13.79.
2.3 General procedure for the synthesis of CdSe MSNCs using
1,2,3-selenadizoles
A series of 1,2,3-selenadiazoles were used for the synthesis of
CdSe MSNCs (A–I). Herein, 1,2,3-selenadiazole and cadmium
acetate Cd(OAc)2 were reacted in a 1 : 1 ratio. Initially, Cd(OAc)2
(1 eq.) and oleic acid (7–10 mL) were refluxed under a nitrogen
atmosphere for 5–10 min to obtain a homogeneous solution.
Subsequently, 1,2,3-selenadiazole (1 eq.) pre-dissolved in diphenyl
ether (30 mL) was added. The reaction mixture was heated
between 140 and 150 1C for 3–5 h, and the reaction progress
was monitored using UV-Visible spectroscopy. Finally, the reaction
mixture was cooled down to room temperature. The mixture
was kept overnight as an aging process by adding n-hexane and
methanol. The precipitate was separated by centrifugation. The
as-synthesized MSNCs were washed with methanol. The isolated
MSNCs were dried in an oven for about 1–2 h between 50 and
70 1C. The yield was typically in the range of 45–55%.
2.2.3. 4-Heptyl-1,2,3-selenadiazole
(4).
4-Heptyl-1,2,3-
selenadiazole (4) was synthesized from 2-nonanone semicarba-
zone. Yield 3.03 g (58%), yellow liquid, Rf 0.56. UV spectrum
3. Results and discussion
(toluene), lmax (nm): 300. IR spectrum, n (cmÀ1): 2853–2951 In general, various parameters, such as the amount of surfactant,
1
(C–H), 1585 (CQC), 1464 (–CH defr.), 1275 (C–N str.). H NMR temperature, and reaction time, affect the formation of the
spectrum, d (ppm), J (Hz): 0.78–0.81 (3H, m, J = 10.0, CH3); MSNCs. By controlling the reaction parameters systematically,
1.19–1.24 (2H, m, J = 10.0, CH3), 2.05–2.08 (2H, m, J = 10.0, the synthesis of CdSe MSNCs with the desired size, shape, and
CH2); 2.32–2.35 (2H, m, J = 10.0, CH2); 8.80 (1H, s, selenadiazole properties is possible.20 In the current study, different 1,2,3-
ring H). 13C NMR spectrum, d (ppm): 13.8 (CH3); 18.4 (CH3); selenadiazole derivatives were utilized for the preparation of
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CdSe MSNCs as a source of selenium. In the recent past, it has the various groups. FTIR spectra of compounds (3, 4, 6, and 8)
been shown that 1,2,3-selenadiazoles are useful selenium pre- (Fig. S1–S4, ESI†) were recorded as KBr pellets. In general, the
cursors due to their thermally labile nature. Among them, peaks observed at 2951–2848 cmÀ1 were due to the C–H
cycloalkeno-1,2,3-selenadiazoles have been exclusively studied stretching vibrations, and the peak at 1648–1627 cmÀ1 was
for the preparation of metal selenide QDs, MSNCs, as well as assigned to the CQC stretching vibrations. The FTIR bands
selenium nanoparticles.21–23 In our recent studies,22a,23b we appearing at 1482–1436 cmÀ1 were assigned to C–H deformation,
have investigated the effect of the amount of capping agent, and the peaks at 1303–1286 cmÀ1 were assigned to C–N stretching.
solvent, and microwave power on the synthesis and properties Additionally, 1,2,3-selenadiazoles bearing aromatic group substi-
of the MSNCs prepared using cycloalkeno-1,2,3-selenadiazoles. tuents showed bands at 3130–3080 cmÀ1 for C–H stretching and
The formation of various-sized nanoclusters of CdSe was con- bands at 1580–1600 cmÀ1 for CQC stretching vibrations. The FTIR
firmed by UV-visible spectroscopy. The study showed that an spectra showed variations in the stretching modes of vibrations
increase in the amount of OLA and microwave power promotes due to the presence of different functional groups in the
the formation of larger nanoclusters. The present article compounds. The interpretation as well as the data matched
addresses the effect of different selenium precursors on the well with the reported values.19,24,25 The 1H NMR spectra of the
synthesis of MSNCs, and for this purpose, different 1,2,3- 1,2,3-selenadiazoles (Fig. S5–S12, ESI†) were obtained in CDCl3.
selenadiazole derivatives have been utilized for the preparation In the 1H NMR spectra of the 1,2,3-selenadiazoles, distinct
of the CdSe MSNCs. Without any size sorting, a single family of signals were obtained for different types of protons, e.g. CH3,
CdSe MSNCs was reproducibly synthesized using the alterna- CH2, and CH, protons, present in the compounds, which showed
tive selenium precursors. During the synthesis, it was observed chemical shifts at d = 1.10–3.25 ppm. The heterocyclic ring protons
that the exclusive formation of MSNCs occurred when the showed chemical shifts in the range d = 8.70–9.50 ppm. Similarly,
aliphatic derivatives of 1,2,3-selenadiazoles were used. However, the chemical shifts in the range of d 6.90–8.3 ppm were assigned to
the use of aromatic derivatives of 1,2,3-selenadiazoles resulted in a the aromatic ring protons in the compounds having aromatic
mixture of regular-sized QDs as well as magic-sized nanoclusters. moieties. The 13C NMR spectra of the 1,2,3-selenadiazoles
In the current study, the precursors, i.e. 1,2,3-selenadiazoles were (Fig. S5–S13, ESI†) revealed the expected number of signals for
synthesized by solventless method to avoid their photochemical different types of carbons in the molecules. The signals at the
degradation during the reactions due to their thermal and photo- chemical shift values at d = 130–137 ppm were assigned to the
chemical sensitivity. Moreover, since the solution processing of CQC–Se carbons, and the signals at d = 157–165 ppm were
selenadiazoles requires a large amount of solvent and lengthy assigned to the CQC–N carbons of the heterocyclic ring depending
work-up, solventless synthesis of a series of cyclic, acyclic aliphatic, on the type of selenadiazole. Mass spectra were obtained
acetophenone, and other aromatic derivatives of 1,2,3-selena- (Fig. S13–S16, ESI†) to indentify the exact mass and further
diazoles was carried out. Overall, the current study describes the confirm the structures. The observed mass values matched well
synthesis of 9 selenadiazole derivatives including 4 novel deriva- with the expected mass of the compounds. A typical molecular
tives (3, 4, 6, and 8) that have not been previously reported to the ion peak was observed in all the cases along with a set of
best of our knowledge. The new selenadiazole derivatives were isotopic components, which was characteristic for the presence
synthesized using the corresponding semicarbazones and sele- of selenium, comprising 6 stable isotopes at an atomic mass
nious acid. Semicarbazones are typical starting materials, which 72–82.24,25 The m/z value of all the compounds corresponded to
have been synthesized in the laboratory using the corresponding the respective protonated molecular ions. CHN analysis was
ketones and semicarbazide (Scheme 1). The synthesized com- carried out to determine the exact elemental composition and
pounds were purified and thoroughly characterized using different purity of the compounds. TGA analysis of the 1,2,3-selena-
spectroscopic tools.
diazoles (Fig. S17, ESI†) was performed to determine the
FTIR spectroscopy was employed to understand the nature thermal behaviour of the compounds. As selenadiazoles are
of various functional groups present in the selenadiazoles. The known to eliminate Se upon thermal decomposition, TGA is an
formation of CQC, C–N, and C–Se bonds along with the important aspect to understand their suitability as selenium
presence of various alkyl or aryl groups in different derivatives precursors such that these can be reacted at the desired tem-
were confirmed by IR spectroscopy. The symmetric and asym- perature for the synthesis of metal selenides. Representative
metric modes of vibrations were correlated with the nature of examples of the TGA curves presented in Fig. S17 (ESI†) show
that the 1,2,3-selenadiazole derivatives decompose in the
temperature range of 100–200 1C. Almost all the compounds
showed a two-step decomposition profile. Aliphatic 1,2,3-selena-
dizoles decompose at a lower temperature (90–300 1C), whereas
the aromatic 1,2,3-selenadiazoles require a higher decomposition
temperature (120–350 1C). Indeed, compound 9 showed a three-
step decomposition profile. The first step degradation is attri-
buted to the elimination of nitrogen molecule from each of
the selenadiazoles. While in the second degradation step, the
aliphatic or aromatic group is decomposed, and simultaneous or
Scheme 1 The solventless synthesis of 1,2,3-selenadiazoles and their
derivatives.
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subsequent decomposition may arise from the vaporization of systematic manipulation and molecular tuning of the precursors
selenium itself (m.p. of Se is B215 1C).26
can be considered as one of the key steps in controlling the final
NC properties and quality.
3.1. Synthesis of magic-sized CdSe clusters
All the CdSe samples synthesized using a variety of selena-
diazoles showed doublets at 370 nm and 390 nm, indicating the
formation of single family MSNCs (A–I) (Fig. 1a–d). The MSNCs
of CdSe derived from the short chain alkyl (A and B) (Fig. 1a1)
and long chain aliphatic alkyl (C and D) (Fig. 1b1) 1,2,3-
selenadiazole derivatives showed sharp doublets without the
signature of other components in their UV-visible spectra.
Since in the literature, a sharp doublet has been correlated
with one type of family of magic cluster,3b,23 this, therefore,
indicates the formation of a single-sized family of MSNCs.
Similarly, MSNCs of CdSe obtained using the acetophenone
derivatives (E and G) of 1,2,3-selenadiazoles showed somewhat
broad absorption bands positioned at about 400 nm and 425 nm
with an additional band at around 550–575 nm (Fig. 1c1).
Similarly, the use of aromatic 1,2,3-selenadiazole derivatives as
the source of selenium resulted in the formation of MSNCs (H, I)
(Fig. 1d1) with an absorption in the region of 370–390 nm with a
broad band at about 525 nm. In these reactions, the formation of
regular sized QDs (RSQDs) along with the MSNCs took place
possibly due to the release of selenium in installments because of
the multi-stage thermal decomposition that occurs during the
reaction. Furthermore, it can be noted that the CdSe MSNCs (E–G)
show a higher red shift in the labs value as compared to the H and
I samples. This suggests that somewhat larger CdSe particles are
present in E–G as compared to those in H and I. The conversion of
the precursor into monomers is considered to be rate determining
step and thus affects the nanocluster properties.4b
The synthesis of MSNCs was carried out under reflux condi-
tions (150 1C) and a N2 atmosphere using diphenyl ether (DPE)
as the solvent and oleic acid as a capping agent (Scheme 2).
Herein, four different series of 1,2,3-selenadiazoles were utilized
as a selenium source: small chain aliphatic, long chain aliphatic,
acetophenone, and aromatic selenadiazole derivatives. The
MSNCs prepared using different selenadiazoles were analyzed
using spectroscopic techniques. The sample codes given for
synthesized CdSe MSNCs using corresponding selenadiazoles
are described in Table 1.
Due to the quantum confinement effect, UV-visible spectro-
scopy allows to determine the size and concentration as well
as predict the size distribution of nanoclusters because the
electronic structure of nanoclusters is strongly sensitive to their
size. The mechanism of the formation of nanoclusters and the
effect of different selenium precursors can be broadly under-
stood by analysis of the UV-visible spectroscopic data. The
precursor reactivity, which controls the monomer formation
rate and thus, the degree of supersaturation, has a strong effect
on the nucleation and growth of colloidal NCs and thus on
their size evolution and final size distribution. Since the pre-
cursor reaction is the starting point of any NC synthesis,
Overall, the effect of the Se precursors (1,2,3-selenadiazoles)
employed on the optical properties can be well understood by
Scheme 2 The synthesis of CdSe MSNCs using 1,2,3-selenadiazoles via a
thermal method.
Table 1 The sample codes used for the CdSe MSNCs (given in the parenthesis) obtained from their corresponding 1,2,3-selenadiazole derivatives
Sr. 1,2,3-Selendiazoles
no. (starting reagents) (1–9) 1,2,3-selenadiazoles) (A–I)
MSNCs (synthesized from corresponding Sr. 1,2,3-Selendiazoles
MSNCs (synthesized from corresponding
no. (starting reagents) (1–9) 1,2,3-selenadiazoles) (A–I)
1
2
CdSe MSNCs (A)
CdSe MSNCs (B)
6
7
CdSe MSNCs (F)
CdSe MSNCs (G)
3
4
5
CdSe MSNCs (C)
CdSe MSNCs (D)
CdSe MSNCs (E)
8
9
CdSe MSNCs (H)
CdSe MSNCs (I)
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The aromatic derivatives generally decompose at higher
temperatures, and the acetophenone derivatives of 1,2,3-selena-
diazoles are more stable among them, as evident from TGA
(Fig. S17, ESI†). The cleavage of these compounds can occur
only at thermodynamically attained kinetics. At this temperature,
an excess release of selenium occurs randomly due to the sudden
cleavage of the molecules, which promotes uncontrolled rapid
nucleation leading to formation of large-sized particles. The
mechanism of formation is given in Scheme 3.
Furthermore, structural comparison of the selenadiazoles
reveals the presence of long conjugation with aromatic double
bonds. The presence of the aromatic ring makes the C–Se bond
much weaker as compared to that of the small and large alkyl
substituents, which are known to possess an electron-donating
effect i.e. +inductive effect. This situation leads to the instant
release of Se at the decomposition temperature.
Therefore, it can be emphasized that the presence of aromatic
rings favours the formation of comparatively larger MSNCs with
or without RQDs. Thus, multiple broad peaks/bands with red-
shifts can be co-related to the surface defects, which can be better
understood by PL. The band gap of these NCs obtained from the
absorption spectra was found to be 3.35 eV when calculated using
the labs = 370 nm values. Similarly, the peaks at 390 nm can also
be utilized for the band gap calculation, and the average band gap
can, therefore, be determined. From the absorption wavelength,
the particle size of the CdSe (MSNCs) was calculated using the
effective mass approximation equation.21,22,27,28 The as-obtained
particle size values in the present cases are reported in Table 2.
The PL spectra of the CdSe MSNCs synthesized via the
thermal methods were also obtained in toluene at lex = 350 nm.
All the samples showed broad emissions, confirming their
white light characteristics (Fig. 1a–d). It has been reported that
MSNCs of CdSe predominantly show a combination of band
edge and broad photoluminescence.3b,13,17a,29,30 According to the
literature, the PL emission in MSNCs is mainly affected by the
presence of non-coordinating selenium sites on their surface that
create energetically distinct mid gap states, a unique structural
Fig. 1 The UV-visible and PL spectra of the CdSe MSNCs (A–I) prepared
from their respective 1,2,3-selenadiazole derivatives.
dividing all the selenadiazole precursors into three categories
based on the attached substituents: small alkyl chain (A and B),
large alkyl chain (C and D), and aromatic ring-containing
selenadiazoles (E–I). From the UV-visible analysis, it was observed
that the small and large alkyl chain-containing 1,2,3-selena-
diazoles (A–D) showed optical behaviour of CdSe MSNCs only
due to being less stable with lower decomposition temperatures
(100–120 1C), whereas the aromatic ring-containing selenadiazoles
showed behaviour of both MSNCs and regular QDs. This can
be considered due to the comparatively higher stability of the
aromatic derivatives (150–180 1C) as well as the instant release of
selenium at their decomposition temperatures leading to rapid
growth and nucleation.
Scheme 3 The mechanism for the formation of CdSe MSNCs using
aliphatic and aromatic derivatives of 1,2,3-selenadizoles.
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arrangement of MSNCs, and the steric effect of the surface
capping ligands. Herein, the broad PL emission was observed
from the MSNC solution.13 In our previous report, we have
described that the capping agent plays a crucial role in defining
the emission as well as the other properties of the MSNCs.23 The
broad PL emission was mainly influenced by the nature of the
capping ligand. The large surface of the NCs smaller than 2 nm
remains mostly un-passivated. This ultimately leads to many
defects in the NCs.
The lem values for each sample are reported in Table 2. The
broad PL profile supports the potential application of these
nanoclusters in white light-emitting devices. The presence
of dangling bonds, various trap states, and possible defects
Fig. 2 The PL lifetime decay analysis of the CdSe MSNCs of (a) sample B
and (b) sample E.
present on the surface result in the multiple peaks observed in ranging from 0.32 to 0.33 nm. These values are well matched for
the PL spectra. Additionally, the PL lifetime decay analysis of CdSe clusters. The values for different samples of CdSe are
some of the samples (4two month old samples) was carried reported in Table 2. A comparative study of the crystallite size
out to understand the non-radiative recombination process as values revealed the formation of MSNCs with smaller sizes
well as to study the presence of the trap states (Fig. 2). The when small and large alkyl chain selenadiazoles were employed
stored samples when rescreened for PL showed marginally as the Se source (A–D), whereas the use of aromatic ring-
red-shifted emissions, and the lifetime decay was accordingly containing selenadiazoles showed the formation of compara-
discussed. The PL lifetime in the CdSe MSNCs (B) was estimated tively larger particles (E–I). This was in good agreement with the
from tavg considering the emission peaks at 389 nm, 400 nm, and red shift obtained in the UV-visible spectroscopy spectra for the
544 nm. The values were found to be 1.20 ns, 3.50 ns, and 11.1 ns, CdSe MSNCs (i.e. in case of E–I).
which were considered based on the literature.23b Similarly, for
sample E, the tavg was found to be 30.52 ns, as calculated from the (small and large alkyl chain selenadiazoles), there was a small
emission peak at 540 nm. signal at 2y about 291, a signature for the presence of traces of
It was noticeable that in the case of sample A, B, C, and D
From the time-resolved spectroscopy spectra, it was realized selenium; however, the same was not displayed prominently
that an increase in the emission wavelength increased the PL when the aromatic ring-containing selenadiazoles (E–I) were
decay times (Fig. 2a). The charge recombination from the used for the synthesis. TEM analysis (Fig. 4) showed the
surface mid-gap states due to the presence of selenium on presence of very small particles in the sub 2 nanometer region.
the surface of the MSNCs leads to multiple emissions.31 The The TEM images were not very resolved due to extremely small
interaction of surface passivating reagents with the magic- particles and the excess capping of oleic acid. It can be seen
clusters may also cause surface-related defects.32 It has been from the TEM images that the morphology appears to be of a
reported that oleic acid shows chemically weak interactions rice-gain type. The selected area electron diffraction (SAED)
with the surface Cd or Se atoms; this leads to a weak physical patterns showed that the samples were slightly crystalline in
adsorption.33,34 The formation of CdSe MSNCs using the thermal nature as the rings associated with the various crystal planes
method was further confirmed by powder XRD analysis (Fig. 3). were clearly visible.
Powder XRD of all the samples (A–I) revealed their cubic (zinc
The size distribution of the particles must be in a narrow
blende) crystal structure with reflections cantered at around 2y range for many useful applications. Indeed, the dynamic light
values of 25, 42, and 511 corresponding to the (110), (220), scattering measurements showed that the particle size distri-
and (311) crystal planes. The crystallite size and lattice spacing bution was in the narrow range with a specific mean diameter
were calculated using the Debye Scherrer equation and Bragg’s of the particle size between 1 and 2 nm (Fig. S18, ESI†). EDAX
equation, respectively. It was estimated that the diameters ranged analysis of four typical samples was carried out, which showed
between 1.23 nm to 1.59 nm with the d values at the (111) planes a non-stoichiometric atomic ratio between Cd and Se for the
Table 2 The analytical data obtained for the CdSe MSNCs prepared using various 1,2,3-selenadiazoles
Absorption
(nm)
Emission (nm)
(l(ex) @ 350 nm)
Crystallite size (nm)
from XRD
‘d’ (nm) from
XRD (for 111)
Particle size
by DLS (nm)
Sample name
A
B
C
D
E
F
G
H
I
370, 390
372, 392
370, 305
370, 390
370, 390, 550
370, 390, 580
370, 390, 570
370, 390, 510
370, 390, 500
410, 525
410, 425, 515
410, 525.
410, 500
450, 540
550
425, 550
500
550
1.23
1.25
1.32
1.41
1.43
1.55
1.59
1.51
1.48
0.33
0.33
0.33
0.33
0.32
0.33
0.33
0.33
0.33
1.2
1.5
1.7
1.55
1.8
1.75
1.50
1.60
1.55
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Fig. 5 EDAX of the CdSe MSNCs (A, C, E and H).
Fig. 3 The XRD patterns of the CdSe MSNCs prepared from (a) aliphatic
short chain (A and B), (b) aliphatic long chain, (c) acetophenone (E and F),
and (d) aromatic (H and I) derivatives of 1,2,3-selenadiazoles.
Fig. 6 The FTIR spectra of the CdSe MSNCs prepared from the different
derivatives of 1,2,3-selenadiazoles (A–I).
suppressed. A peak at around 1525–1530 cmÀ1 in the magic
clusters can be assigned to asymmetric COO– stretching
vibrations.
Fig. 4 TEM images (inset SEAD) of the CdSe MSNCs (A, C, E and H).
The Raman spectra of the selected MSNCs are shown in
Fig. 7. The Raman spectra were obtained to determine the
various Raman active vibrations in the nanoclusters due to the
sample prepared using aliphatic selenadiazoles with a higher various phonon modes and functional groups present in the
selenium content; this was also evident in the XRD results. capping ligands. In general, the CdSe NCs show Raman scattering
An almost matching atomic ratio of 1 : 1 (Cd : Se) was, however, bands for longitudinal optical (LO) and transverse optical (TO)
present when the CdSe clusters were synthesized using the phonons as well as their overtone vibrations. The Raman spectra
other selenadiazole derivatives (Fig. 5).
The FTIR spectra of all the MSNCs showed the presence of 350 cmÀ1 and 1690 cmÀ1
typical oleic acid peaks (Fig. 6). The FTIR spectra of oleic acid
In nano-sized metal chalcogenides, the LO phonon modes
and the CdSe MSNCs showed characteristic peaks in the are reported to be size dependent mainly due to lattice
region of 2850–2955 cmÀ1 corresponding to the CH2 stretching contractions.35,36 In Fig. 7, the peaks at 350 cmÀ1, 540 cmÀ1
of the CdSe MSNCs (Fig. 6) gave rise to several peaks between
.
,
vibrations. These three peaks were observed at about 2850, and 620 cmÀ1 are due to the LO phonons, and the peak at
2925, and 2955 cmÀ1. Due to its CQO stretching vibrations, around 865 cmÀ1 is due to their overtone vibrations. Similarly,
oleic acid shows a peak at 1710 cmÀ1. This peak in the CdSe the peaks at around 1275 cmÀ1, 1500 cmÀ1, and 1690 cmÀ1 are
MSNCs was almost diminished and observed to be highly due to the functional groups present in oleic acid. C–O–C str. is
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growth of single family of MSNCs reported herein is of general
interest and can also be potentially extended to other metal
selenide nanoparticles. The current study thus highlights an
important synthetic approach to prepare only single family MSNCs.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are thankful to the vice-chancellor, DIAT for the
support. PKK thanks DST, Govt of India, for the research grant
(No. SR/S1/PC-39/2010). The authors thank Mr Anuraj Kshirsagar
for his fruitful discussions.
Fig. 7 The Raman spectra of the CdSe MSNCs obtained from their
corresponding 1,2,3-selenadiazole derivatives (A, C, E and H).
observed at 1275 cmÀ1, the CH deformation peak is observed at
1500 cmÀ1, and the peak observed at 1690 cmÀ1 is due to CQO
stretching mode of vibration.35,36
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