Multilayers of CdSe Nanocrystals and Bis(dithiocarbamate) - American

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Multilayers of CdSe Nanocrystals and Bis-Dithiocarbamate Linkers Displaying Record Photoconduction Gianni Zotti, Barbara Vercelli, Anna Berlin, and Tersilla Virgili J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 19 Nov 2012 Downloaded from http://pubs.acs.org on November 19, 2012

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The Journal of Physical Chemistry

Multilayers of CdSe Nanocrystals and Bis-dithiocarbamate Linkers Displaying Record Photoconduction. G. Zotti* Istituto CNR per l’ Energetica e le Interfasi c.o Stati Uniti 4, 35127 Padova (Italy) B. Vercelli Istituto CNR per l’ Energetica e le Interfasi Via R. Cozzi 53, 20125 Milano (Italy) A. Berlin Istituto CNR di Scienze e Tecnologie Molecolari via C. Golgi 19, 20133 Milano (Italy) T. Virgili Istituto CNR di Fotonica e Nanotecnologie P.zza Leonardo da Vinci 32, 20132 Milano (Italy)

dr. Gianni Zotti Istituto CNR per l’ Energetica e le Interfasi C.o Stati Uniti 4 35127 Padova (Italy) tel (39)049-829-5868 fax (39)049-829-5853 e-mail [email protected]

ACS Paragon Plus Environment

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Abstract

Bis-dithiocarbamate molecules were reacted with (hexadecylamine/stearate)-capped CdSe nanocrystals (7.5 nm diameter) to form multilayers on ITO glass via layer-by-layer alternation. The new materials were investigated by UV-vis and FTIR spectroscopy, photoluminescence and photoconductivity. FTIR and UV-vis analysis of multilayers showed that the bis-dithiocarbamate linkers operate the complete removal of caps and cover the nanocrystal layer (ca 2x10-8 mol cm-2 layer-1 in CdSe units) with a dense linker layer (ca 2x10-9 mol cm-2 layer-1). The photoconductivity of the multilayer films obtained with such linkers is strongly enhanced, compared with analogous films produced with biscarboxylate linkers of comparable length and structure, attaining record values 10 to100 times higher.

Keywords:

photoconductivity,

photoluminescence,

nanocrystal,

dithiocarbamate,

carboxylate, multilayer.


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1. Introduction Organic ligands typically used in nanocrystal synthesis cause very poor interparticle contacts. Highly photoconductive films of CdSe nanocrystals (CdSe-NCs) may in any case be prepared by exchanging the original bulky long-chain ligands with appropriate linkers. While much effort has been placed on optimizing molecular core conductivity, there have been relatively few attempts at designing optimal linker groups to semiconducting nanocrystals. Thus commonly used end groups are anions such as carboxylate, sulfonate or phosphonate and, with a lower strength, neutral nucleophiles such as pyridines and amines. In respect to this issue the carbodithioate (and dithiocarbamate DTC, as a particular case) moiety is a special case. 1 Recent production2 of various DTC self-assembly on gold surface have opened the use of DTC linkers to anchor molecular wires to gold electrodes. It was then demonstrated theoretically (by computation)3 that the stronger molecule-electrode coupling associated with the conjugated DTC linker, with the extension of the π-conjugation from the molecule to the gold electrodes, leads to enhanced electrical conductance. Single-molecule conductances in phenylene-ethynylene molecules terminated with carbodithioate linkers4 demonstrate that the carbodithioate linker increases electronic coupling to the metal electrode and lowers the effective barrier for charge transport relative to thiol linkers. Moreover, the optical and electrical properties of films composed of 4 nm gold nanoparticles (AuNPs) interlinked by bis-DTC derivatives have given more support.5 Films prepared with the thiol-terminated linker molecules exhibit thermally activated charge transport and a regular plasmon band around 550 nm. The AuNPs/bis(dithiocarbamate) structures have a significantly red-shifted plasmon band (at ca 630 nm) and a very low activation energy for charge transport. These differences are explained in terms of the formation of a resonant state at the interface due to overlap of the molecular orbital and metal wave function, leading to an apparent increase in nanoparticle diameter.5 Only quite recently the carbodithioate system has been applied to CdSe-NCs. In fact, dithiocarbamates have been used as ligands for stabilizing core-shell CdSe-NCs.6 Surface functionalization of CdSe-NCs was first reported with alkyl or aryl derivatives of carbodithioic acids exchanging the initial surface ligands under mild conditions.7 Later a series of regioregular oligo- and polythiophenes containing carbodithioate groups were synthesized.8 ACS Paragon Plus Environment

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More recently it has been reported that coordination of phenyldithiocarbamate ligands to CdSe-NCs in solution decreases their optical band gap by a record value of 0.2 eV, corresponding to an apparent increase in the excitonic diameter of 0.5 nm.9 The HOMO of phenyldithiocarbamate is near resonant with that of the NC, and the two have correct symmetry to exchange electrons. It is therefore expected that the relaxation of exciton confinement through delocalization of the photoexcited hole of the NC into the ligand shell will favor charge transport in such systems. Last we mention the recent synthesis of coordination polymers10 and dimers11 via the 1,4benzenedicarbodithioate dianion as linker, pointing to the development of metal-organic solids with electronic properties similar to those found in pure inorganic chalcogenidebased materials. As a continuation of our previous work with other linkers,1,12-14 this paper reports on the replacement of bulky surface ligands, capping spherical CdSe-NCs, by bis-DTC molecules, such as the aliphatic ethylene-1,2-bis(dithiocarbamate) (EDTC) and the aromatic 1,4-phenylene-bis(dithiocarbamate) (PDTC) (shown in Chart 1), in the formation of hybrid multilayers by the layer-by-layer (LBL) method. The aim of this work is to verify if the photoconductivity of these multilayer thin films is favoured by the presence of conjugated linkers present in DTC molecules. To this end results are compared with those from layers produced similarly with bis-carboxylate linkers of comparable length and structure such as adipic acid CAC4CA (for EDTC), and 1,4-phenylenediacetic acid CACH2PhCH2CA (for PDTC) (see Chart 1).


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2. Experimental Section

2.1. Chemicals and Reagents. Disodium 1,2-ethylene-bis(dithiocarbamate) (EDTC), adipic acid (CAC4CA), 1,4-phenylenediacetic acid (CACH2PhCH2CA), poly(acrylic acid) (PAAH), and all other chemicals were reagent grade and used as received. Disodium 1,4phenylene-bis(dithiocarbamate) (PDTC) was prepared as reported in the literature.5 Soluble CdSe-NCs with the surface capped by hexadecylamine and stearic acid were produced as previously reported.1 The NCs display an absorption maximum at 645 nm corresponding to an average size of 7.5 nm.15 CdSe-NCs were used ca 10-2 M (in CdSe units) in CHCl3. 2.2. Substrates and Multilayer Film Formation. Transparent conducting surfaces (1x4 cm2) were prepared from indium-tin-oxide (ITO)/glass (20 Ω sq-1 from Kintec, Hong-Kong). The build-up of multilayers was performed according to the layer-by-layer (LBL) methodology, i.e. by dipping the substrate alternatively into the solutions of the two components. After each immersion step the substrate was carefully washed and dried in air. Multilayers were built on ITO (coated with a PAAH monolayer from a 10-3 M solution of PAAH in EtOH1) via alternation of CdSe-NCs 10-2 M in CHCl3 and the linker 10-3 M in MeOH. Exposing time was 5 minutes. Adsorption occurs equally on both sides of the ITO/glass substrate. 2.3. Apparatus and Procedure. UV-vis spectra were run on a Perkin-Elmer Lambda 15 spectrometer; FTIR spectra were taken using a Perkin-Elmer 2000 FTIR spectrometer. Multilayer thicknesses were determined with an Alphas-step IQ profilometer from KLA Tencor. Photoluminescence experiments were carried out at room temperature. The sample was optically pumped by the second harmonic (400 nm, duration pulse of around 50 fs) of a Ti:Sapphire femto-second laser system. The incident pulse energy was ca 1 mJ. A fibre bundle was placed close to the sample to collect the emitted light. Detection was performed using an Oriel Instaspec IV spectrometer with 1 nm spectral resolution. Photoconductivity measurements of multilayers were performed with a special Hg electrode contacting the multilayer-covered ITO as described previously.1 Bias was applied to ITO vs Hg electrode. Illumination was performed on the back glass side of the ITO/multilayer with a water-filtered 100 W halogen lamp, spotted over an area of ca 10 cm2. The resulting light power, calibrated with a silicon photodiode, was ca 100 mW cm-2.


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3. Results

3.1. CdSe/linker Bulk Polymers. Addition of excess DTC linker (carboxylate linkers were reported previously12) dissolved in tetrahydrofuran to the CdSe-NC dispersion in chloroform causes the precipitation of aggregated NCs. After stirring overnight, the insoluble material has been filtered off, washed carefully with CHCl3 and MeOH to remove completely unreacted materials and dried under vacuum. FTIR analysis of the aggregates as KBr pellet (Figure 1) has shown that upon treatment of the nanoparticles the starting bands are strongly decreased, revealing an extensive substitution of hexadecylamine and stearate ligands. New bands are present instead, namely two dominant bands at 1155 and 1007 cm-1 (asym and sym C=S stretching modes) with EDTC. PDTC has produced the corresponding bands at 1130 and 1010 cm-1. Extensive substitution was also observed with carboxylate1 and carbidithioate linkers7. The persistence of CH2 stretching bands around 2900 cm-1 suggests in fact that the originally present ligands are to some extent kept. This is anyway not the case with the multilayers reported below. 3.2. CdSe/linker Multilayers. Multilayers are built with alternate exposure to linker solutions in MeOH and CdSe-NC dispersions in CHCl3. The ligand exchange carried out at room temperature for some minutes on the layered CdSe-NCs leads to a nearly quantitative replacement of the initial surface ligands, as evidenced by surface FTIR on gold substrates, following the previously reported method.14 Multilayers appear very uniform under SEM examination, although there is no apparent difference with bare ITO. In fact the multilayers are thin if compared with the ITO grain size. Moreover the multilayers are robust enough to stand a standard sticky-tape test with no appreciable loss. UV-visible spectroscopy was used to monitor the assembling process of such LBL film on ITO substrates and the obtained optical and thickness parameters are summarized in Table 1. The absorption spectra of multilayer films prepared with different numbers of layers is shown in Figure 2. The observed linear increase of CdSe absorbance (at 645 nm, CdSe first exciton band) vs the number of layers indicated a stepwise and uniform assembly process. The two-side absorbance differential increase (∆ACdSe) depends on the nature of the functional group of the linker. DTC-based linkers present ∆ACdSe around 10 x 10-3 au layer-


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1

(corresponding to ca 2x10-8 mol cm-2 layer-1 in CdSe units15) while for carboxylate linkers

the values are around 6 x 10-3 au layer-1. The thickness of a 10-multilayer, abbreviated as (CdSe/linker)10, depends on the length of the spacer chain between linker’s functional groups. In fact, thickness is 60 nm for ethylene and 70 nm for phenylene spacers, reflecting the higher length and rigidity of the latter. In any case the reported thickness values compared with the optical growth, confirm a compact structure of the multilayer itself. The linkers do not show up in the UV-vis spectrum, due to their high energies of absorption beyond the limitations given by the ITO/glass substrate. Yet the linker coverage could be obtained for PDTC dissolving the multilayer in concentrated hydrochloric acid and measuring the absorbance at the maximum wavelength of absorption (295 nm). From the extinction coefficient of PDTC (15,000 M-1 cm-1) the results correspond to ca 2x10-9 mol cm-2 layer-1 , i.e. to a dense coverage of the CdSe-NCs, like those previously produced with e.g. diamine linkers.14 3.3. PL Spectroscopy. PL spectra of the DTC multilayers are shown in Figure 3. The PL emission at 650 nm, involving only the inorganic component of the multilayer, is weak. DTC linkers show a quenching effect on PL, as reported in the literature.7 In particular in PDTC multilayers, PL is almost totally suppressed (Figure 3). This result may be explained by a faster charge transfer between nanocrystals through the conjugated aromatic ring, which inhibits radiative recombination of the excitons photocreated on the nanocrystal.7 3.4.Photoconductivity. The PL quenching action of linker molecules is a favourable condition for photoconductivity, which in fact reaches very high values. Since the multilayer, even noticeably thick (80 nm) and hole-free (by AFM analysis), could not stand gold coating by evaporation without forming electrical shorts, photoconductivity measurements have been performed using Hg junctions. These junctions neither damage the organic ligands on the NCs nor form persistent metal columns that short the junction. Hg is particularly suited as an electrode for use with CdSe-NCs and for such a contact we have routinely applied a positive bias to the ITO contact. Under the applied conditions (1 V bias and 100 mW cm-2 illumination) thin cast films of the original CdSe-NCs do not show any response. On the contrary the (CdSe/linker)10 layers display photocurrent transients with a response time of some seconds (see e.g. Figure 4). The response is stable for several minutes and at least 5-10 on-off cycles. Table 1 summarizes the limiting steady-state photocurrent values iph for multilayers of the different linkers.


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In general dithiocarbamate-bearing multilayers present iph values which are 1 to 2 orders of magnitude higher than the respective carboxylate-bearing ones. In particular the multilayer (CdSe/PDTC)10 shows the best value, much higher than all the iph values reported in our previous works.1,

12-14

. It appears that the DTC moiety interacts strongly with the NC

surface, particularly through (the conjugation of) the phenylene spacer, favoring charge transport between CdSe-NCs. Photoconductivity measurements have been previously performed on multilayers constituted of CdSe-NCs alternated with different polymers and some related bipolar amphiphiles. The series comprised sulfonic, phosphonic and carboxylic acids; pyridineand amine-based molecules.1,

14

Photoconductivity was observed in all cases with the

exception of pyridine-based polymers. The sulfonate, phosphonate and carboxylate spacers as well as amine linkers were found to be appreciable photoconductors. The photoconductivity of these multilayers is in any case much lower than that we have found in (CdSe/DTC)10 multilayers of comparable thickness. The relationship between photoconduction and thickness has been also investigated but only a limited range of thickness (20-50 nm) was considered. In fact thinner films are exposed to high risks of shorts whereas thicker films are difficult to be produced with regular features. The result is that thicker layers are less photoconducting and it appears that the photocurrent response is proportional with the inverse of thickness. This can be accounted for by a resistance proportional with thickness (ohmic response) in a uniform distribution of photogenerated carriers. The photocurrent transient (Figure 4) shows an exponential saturation during illumination then decays in the dark following a pseudo-first order decay law. Such a behaviour must be attributed to the action of surface traps.16 Unpassivated states of the NC surface may trap holes and when one charge carrier is trapped, the other runs until it recombines with a trapped hole.17 After the illumination is switched off, stopping photocarrier generation, the trapper carriers decay following this recombination pathway. The recombination lifetime is limited by the release rate of the trap and this results in slow response times, frequently higher than some seconds,16 which is our actual case. In fact, photoluminescence measurements have been used to elucidate the relaxation and recombination dynamics of excitons in CdSe-NCs with organic ligands.18, 19 With adsorbed hole acceptors, namely sulfur compounds like hexadecanethiol and phenothiazine, the nanocrystals undergo exciton quenching by hole transfer to surface states, leaving the electron in the conduction band. This appears to be the case also in our systems.


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4. Discussion The donor atoms in the head of organic linkers of CdSe-NCs are usually hard (oxygen or nitrogen1), resulting in strong electronic barriers between the metal center and the rest of the linker. Therefore, linkers with chalcogenide donor atoms, such as sulfur, are in fact promising alternatives. Substitution of oxygen by sulfur decreases the LUMO energy of the ligand, since C-S bonds are weaker than C-O bonds, and increases the HOMO energy because sulfur is less electronegative than oxygen.20 This leads to enhanced electronic communication between ligand and metal centers. The strong PL-quenching and high photoconductivity presented by CdSe/DTC multilayers indicate that the presence of DTC sulfur atoms in the ligand head plays an important role in the mechanisms of both exciton recombination and charge transport. A conjugated system like the phenylene moiety in PDTC produces significant improvements. In fact DFT calculations performed on CdSe-NCs capped by PDTC linkers suggest that such coordination decreases the confinement energy of the exciton through delocalization of excited electron over the ligand.9 Resonance leads to mixing of the HOMOs of the NCs with those of PDTC. The delocalization of the exciton into the ligand shell is facilitated by the conjugation of sulfur atoms of the headgroup with the nitrogen atom.9 In a recent work, optical and electrical properties of films composed of gold nanoparticles interlinked by dithiol or bis-DTC derivatives were compared in order to investigate how these properties depend on the core of the linker molecule (benzene or cyclohexane) and its metal-binding substituents (thiol or dithiocarbamate).5 The film prepared with phenylene-bis(dithiocarbamate) exhibits metallic properties, indicating the full extension of the electron wave function between interlinked nanoparticles. In all cases, replacement of the benzene ring with a cyclohexane ring in the center of the linker molecule leads to a one order of magnitude decrease in conductivity. In another previous paper metal-molecule-metal junctions were fabricated by contacting Au-supported alkyl or benzyl thiol self-assembled monolayers (SAMs) with an Au-coated atomic force microscope (AFM) tip.21 It was found that the resistance of junctions based on benzyl thiol SAMs are more than 10 times smaller than junctions based on alkane thiol SAM of comparable length. A qualitative explanation of this result was based on the different EHOMO-LUMO of the phenyl ring (ca 4 eV) and of the alkyl chain (ca 8 eV).


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These results are in agreement with our photocurrent data on EDTC and PDTC samples, where by changing an ethylene moiety with a phenylene one causes a marked (fivefold) increase of the photocurrent.

5. Conclusions Replacement

of

bulky

surface

ligands,

capping

spherical

CdSe-NCs,

by

bis-

dithiocarbamate molecules strongly enhances the photoconductivity of films from such linkers. Compared with layers produced with bis-carboxylate linkers of comparable length and structure. the photoconductivity attains record values up to 100 times higher when 1,4phenylene-bis(dithiocarbamate) is used. The produced hybrid structures, constituted by semiconductor nanocrystals and organic linkers, easy processable from solution and with a relatively low material cost, may be of high interest for the production of efficient solar cells.


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Table 1 Two-side CdSe exciton-band differential absorbance (∆A); total thickness (d) and solidstate photocurrent (iph) of (CdSe/linker)10 multilayers on ITO from different linkers. _______________________________________________________________________ linker

∆A/10-3 au layer-1

d/nm

iph/mA cm-2

_______________________________________________________________________ EDTC

9 (±1)

60 (±5)

20 (±2)

CAC4CA

6 (±1)

60 (±5)

1.5 (±0.2)

PDTC

11 (±1)

70 (±5)

100 (±10)

CACH2PhCH2CA

7 (±1)

70 (±5)

0.5 (±0.1)

_______________________________________________________________________


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References

(1)

Zotti, G.; Vercelli, B.; Berlin, A.; Chin, P. T. K.; Giovanella, U., Chem. Mater. 2009, 21, 2258-2271 and references therein.

(2)

Zhao, Y.; Perez-Segarra, W.; Shi, Q.; Wei, A., J. Am. Chem. Soc. 2005, 127, 73287329.

(3)

Li, Z.; Kosov, D. S., J. Phys. Chem. B 2006, 110, 9893-9898

(4)

Xing, Y.; Park, T. H.; Venkatramani, R.; Keinan, S.; Beratan, D. N.; Therien, M. J.; Borguet, E., J. Am. Chem. Soc. 2010, 132, 7946-7956.

(5)

Wessels, J. M.; Nothofer, H.-G.; Ford, W. E.; von Wrochem, F.; Frank Scholz, F.; Vossmeyer, T.; Schroedter, A.; Weller, H.; Yasuda, A., J. Am. Chem. Soc. 2004, 126, 3349-3356.

(6)

Dubois, F.; Mahler, B.; Dubertret, B.; Doris, E.; Mioskowski, C., J. Am. Chem. Soc. 2007, 129, 482-483.

(7)

Querner, C.; Reiss, P.; Bleuse, J.; Pron, A., J. Am. Chem. Soc. 2004, 126, 1157411582.

(8)

Querner, C.; Benedetto, A.; Demadrille, R.; Rannou, P.; Reiss, P., Chem. Mater. 2006, 18, 4817-4826.

(9)

Matthew, T.; Frederick and Emily A. Weiss,F.; Weiss, E. A., ACS Nano 2010, 4, 3195-3200.

(10)

Neofotistou, E.; Malliakas, C. D.; Trikalitis, P. N., Inorg. Chem. 2007, 46, 8487-8489.

(11)

Han, M. J.; Liu, C. L.; Tian , P. F., Inorg. Chem. 2009, 48, 6347-6349.

(12)

Zotti, G.; Vercelli, B.; Berlin, A.; Pasini, M.; Nelson, T. L.; McCullough, R. D.; Virgili, T., Chem. Mater. 2010, 22,1521-1532.

(13)

Vercelli, B.; Zotti, G.; Berlin, A.; Pasini, M.; Natali, M., J. Mater. Chem. 2011, 21, 8645-8652.

(14)

Vercelli, B.; Zotti, G.; Berlin, A.; J. Phys. Chem. C, 2011, 115, 4476–4482.

(15)

Yu, W. W.; Qu, L.; Guo, W.; Peng, X.; Chem. Mater. 2003, 15, 2854-2860.

(16)

Bube, R. H., “Photoconductivity of Solids”, Wiley, New York, 1960.

(17)

Porter, V. J.; Geyer, S.; Halpert, J. E.; Kastner, M. A.; Bawendi, M. G., J. Phys. Chem. C 2008, 112, 2308.

(18)

Jiang, Z.-J.; Leppert, V.; Kelley, D. F., J. Phys. Chem. C 2009, 113, 19161–19171.

(19)

Jiang, Z.-J.; Kelley, D. F., J. Phys. Chem. C 2010, 114, 17519–17528.

(20)

Chisholm, M. H.; Patmore, N. J., Dalton Trans. 2006, 3164-3169.


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(21)

Wold, D. J.; Frisbie C. D., J. Am. Che. Soc. 2001, 23, 5549-5556.


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S S

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O N

N

S

O

O S

O adipate (CAC4CA)

1,2-ethylene-bis(dithiocarbamate) (EDTC)

O S

O S

N

N

S

O O

S 1,4-phenylene-bis(dithiocarbamate) (PDTC)

1,4-phenylenediacetate (CACH2PhCH2CA)

Chart 1


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Absorbance (a.u.)

(a)

1,0

0,5

0,0 3000

-1

ν (cm )

2000

1000

2000

1000

0,2

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

0,1

0,0 3000

-1

ν (cm )

Figure 1 – FTIR spectra in KBr pellet of (a) hexadecylamine/stearate stabilized CdSe-NCs and (b) CdSe/EDTC bulk polymer.


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(a)

0,4

0,2

0,0 400

600

λ/nm

800

100 (b)

-3

Absorbance/10 (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Absorbance (a.u.)


50

0 0

2

4 6 8 Number of layers

10

Figure 2 – (a) UV-vis spectra of ITO/(CdSe/EDTC)n multilayers (n = 1-10) and (b) relevant plot of CdSe absorbance (at 645 nm) vs number of layers. Spectra are backgroundcorrected.


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1,0 Photoluminescence (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0,5

0,0 650

675

700

725

750

λ /nm

Figure

3

–

Photoluminescence

spectra

of

ITO/(CdSe/EDTC)10

(black)

and

ITO/(CdSe/PDTC)10 (red) multilayers under the same conditions.


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light off -2

Photocurrent (mA cm )

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20

10 light on

0 0

20

40 Time (s)

60

80

Figure 4 - Solid-state photocurrent transient of ITO/(CdSe/EDTC)10 multilayers at 1 V applied voltage and 100 mW cm-2 illumination


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Multilayers of CdSe Nanocrystals and Bis-dithiocarbamate Linkers Displaying Record Photoconduction. G. Zotti*, B. Vercelli, A. Berlin, T. Virgili


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Multilayers of CdSe Nanocrystals and Bis(dithiocarbamate) - American

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