New Forms of CdSe: Molecular Wires, Gels, and Ordered Mesoporous Assemblies

Article abstract of DOI:10.1021/jacs.6b10077

This work investigates the structure and properties of soluble chalcogenidocadmates, a molecular form of cadmium chalcogenides with unprecedented one-dimensional bonding motifs. The single crystal X-ray structure reveals that sodium selenocadmate consists of infinite one-dimensional wires of (Cd₂Se₃)_n^2n- charge balanced by Na⁺ and stabilized by coordinating solvent molecules. Exchanging the sodium cation with tetraethylammonium or didodecyldimethylammonium expands the versatility of selenocadmate by improving its solubility in a variety of polar and nonpolar solvents without changing the anion structure and properties. The introduction of a micelle-forming cationic surfactant allows for the templating of selenocadmate, or the analogous telluride species, to create ordered organic-inorganic hybrid CdSe or CdTe mesostructures. Finally, the interaction of selenocadmate “wires” with Cd²⁺ ions creates an unprecedented gel-like form of stoichiometric CdSe. We also demonstrate that these low-dimensional CdSe species show characteristic semiconductor behavior, and can be used in photodetectors and field-effect transistors.

Full text of DOI:10.1021/jacs.6b10077

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Article
New forms of CdSe: molecular wires, gels,
and ordered mesoporous assemblies
Margaret H. Hudson, Dmitriy S Dolzhnikov, Alexander S. Filatov, Eric M.
Janke, Jaeyoung Jang, Byeongdu Lee, Chengjun Sun, and Dmitri V. Talapin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b10077 • Publication Date (Web): 01 Feb 2017
Downloaded from http://pubs.acs.org on February 2, 2017
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New forms of CdSe: molecular wires, gels, and ordered mesoporous
assemblies
†
†
†
†
Margaret H. Hudson, Dmitriy S. Dolzhnikov, Alexander S. Filatov, Eric M. Janke, Jaeyoung
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†
,#
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§
*,†,‡
Jang, Byeongdu Lee, Chengjun Sun, Dmitri V. Talapin
†
Department of Chemistry and James Franck Institute, the University of Chicago, IL 60637, USA
Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA
Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA
§
‡
*
Eꢀmail: dvtalapin@uchicago.edu
Abstract
This work investigates the structure and properties of soluble chalcogenidocadmates, a molecular
form of cadmium chalcogenides with unprecedented oneꢀdimensional bonding motifs. The single
crystal xꢀray structure reveals that sodium selenocadmate consists of infinite oneꢀdimensional
2nꢀ
+
wires of (Cd Se ) charge balanced by Na and stabilized by coordinating solvent molecules.
2
3 n
Exchanging the sodium cation with tetraethylammonium or didodecyldimethylammonium
expands the versatility of selenocadmate by improving its solubility in a variety of polar and
nonpolar solvents without changing the anion structure and properties. The introduction of a
micelleꢀforming cationic surfactant allows for the templating of selenocadmate, or the analogous
telluride species, to create ordered organicꢀinorganic hybrid CdSe or CdTe mesostructures.
2+
Finally, the interaction of selenocadmate “wires” with Cd ions creates an unprecedented gelꢀ
like form of stoichiometric CdSe. We also demonstrate that these lowꢀdimensional CdSe species
show characteristic semiconductor behavior, and can be used in photodetectors and fieldꢀeffect
transistors.
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1
. Introduction
Semiconductors play many important roles in modern informationꢀ and energyꢀrelated
technologies. The ability of semiconducting materials to manipulate electrical charges and
absorb or emit photons is utilized in electronic circuits, solar cells, lightꢀemitting diodes, and
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many other devices. IIꢀVI semiconductors, the materials combining Zn, Cd, or Hg with
chalcogens (O, S, Se, or Te), represent one of the most important semiconductor families and are
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ꢀ3
4ꢀ5
6ꢀ7
widely utilized in solar cells, photocatalysts, IR detectors, and other technologies. In
recent years, significant progress has been achieved in the development of lowꢀdimensional IIꢀVI
8ꢀ9
10ꢀ12
semiconductors in the form of quantum dots, quantum rods,
and quantum wells
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3ꢀ14
(
nanoplatelets)
biotags and display technologies.
emerged as the most studied material and has been used as a model system in many optical
that exhibit excellent light emission properties and are commercially used for
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5ꢀ16
Among lowꢀdimensional semiconductors, CdSe has
17ꢀ18
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9ꢀ20
and electronic studies
of lowꢀdimensional semiconductors. In this work we report three new
forms of lowꢀdimensional CdSe: molecular polymers, gels, and mesoporous assemblies.
The solution synthesis of lowꢀdimensional materials typically involves chemical
transformations of molecular species into a crystalline lattice through nucleation and growth
steps. Currently, we do not have a good understanding of the mechanism of such
transformations, and the structures of intermediate species remain to be revealed. Various
solvothermal methods with metal salts and elemental chalcogens have been used to create
species with one, two, and threeꢀdimensional metal chalcogenide bonding frameworks with
2
1ꢀ27
interesting morphologies and properties.
These approaches typically build up segments of
metal chalcogenide lattice from molecular precursors. Interestingly, lowꢀdimensional metal
chalcogenides can be also accessed through disassembly of the bulk crystalline lattice. For
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example, Mitzi and coworkers utilized the reducing power of hydrazine to dissolve bulk metal
chalcogenides in an excess of chalcogen at room temperature to generate soluble
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chalcogenidometallates.
Mitzi’s approach produced chalcogenidometallates of Sn (S, Se,
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Te), Cu (S, Se), Ge (S, Se), In (Se, Te), and Zn (Te), but soluble products for many
technologically important metal chalcogenides could not be obtained with this approach. We
recently reported the synthesis of soluble chalcogenidometallate species for Cd, Pb, and Bi
chalcogenides by combining strongly reducing chalcogenide salts (Na Ch, K Ch) with bulk
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metal chalcogenide in hydrazine. These complexes were explored for their utility as “solders”
for IIꢀVI semiconductors. For example, a solution of the chalcogenidometallate could be mixed
with ballꢀmilled semiconductor powder and heated to 300 °C to form a polycrystalline film with
good mechanical stability and electrical properties. These chalcogenidometallates also showed
remarkable success as inorganic capping ligands for quantum dots; nanocrystals could be capped
with compositionꢀmatched ligands and annealed at 300 °C to form polycrystalline metal
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ꢀ1 ꢀ1
chalcogenide films. Electron mobilities above 300 cm V s were obtained for annealed films of
CdSe nanocrystals capped with sodium selenocadmate, achieving half the mobility of single
crystalline CdSe and far surpassing previously obtained mobilities for solutionꢀprocessed
36ꢀ37
films.
2nꢀ
In this study we report the structure of a polymeric form of [Cd Se ] which represents,
2
3 n
to the best of our knowledge, the first molecular motif with a oneꢀdimensional CdSe bonding
2nꢀ
framework. The negative charge of the [Cd Se ] chain can be counterꢀbalanced with various
2
3 n
inorganic or organic cations. The choice of a cation can be used to tune the solubility of
2nꢀ
38ꢀ42
[
Cd Se ] wires in polar and nonꢀpolar solvents or to form a micellar template
to generate
2
3 n
mesostructured CdSe and CdTe. We also show that these molecular “wires” can be crossꢀlinked
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2+
with Cd ions to create yet another new form of CdSe, a stoichiometric CdꢀSe gel. This
chemistry can be, at least partially, transferred to other IIꢀVI semiconductors and brings new
opportunities for solutionꢀbased processing of technologically important semiconductors.
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. Experimental Section
All manipulations were performed in a nitrogen glove box with <10 ppm oxygen and water
levels, unless otherwise noted. Further experimental details are given in the Supporting
Information.
43
Synthesis of Na Cd Se . 0.25 mmol Na Se was combined with 96 mg (0.5 mmol) CdSe powder
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2
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2
in 10 mL hydrazine. This mixture was stirred at room temperature until colorless and the dark
solid dissolved, about 5 days. The solution was filtered through a 0.45 µm PTFE syringe filter to
remove any unreacted solids. The product was isolated by adding acetonitrile as nonsolvent and
centrifuging to isolate a white solid. The solid was dissolved in hydrazine at concentrations up
to ~0.05 M.
Growth of Na Cd Se ꢀ 9.5 N H crystals. Single crystals of Na Cd Se were grown by slow
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evaporation of the hydrazine solvent. A solution of ~0.0125 M Na Cd Se in hydrazine was
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loaded into a clean, dry glass tube in the glove box. The tube had a 100° bend in the midsection
and solution was loaded to fill one side of the bend halfway (~0.5 mL). The tube was
temporarily sealed under nitrogen, removed from the glove box, and flame sealed under mild
vacuum. The side of the tube containing the solution was suspended over a warm hot plate (80
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°
C) and the other side was at room temperature. Over the course of about 2 weeks, the hydrazine
evaporated from the warm side and collected in the cool side, leaving behind plateꢀlike crystals.
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+
NEt cation exchange. To create (NEt ) Cd Se , Amberlyst 15 highly acidic cation exchange
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resin was loaded with the desired cation and mixed in 10x excess with Na Cd Se . 1 mg resin
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was rinsed with 3 mL ~1.5 M (NEt )OH in methanol 3 times and then dried under vacuum at 60
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C for an hour. After cation exchange, any excess resin was removed by filtering the solution
through a 0.45 µm PTFE syringe filter.
Using Na Cd Se as ligands for colloidal CdSe nanocrystals. CdSe nanocrystals were
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synthesized using a literature procedure.
were washed an additional 5 times with methanol and 4 times with ethanol in a nitrogen glove
box. Ligand exchange from ODPA to Na Cd Se or (NEt ) Cd Se was performed with slight
For ligand exchange experiments, nanocrystals
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modifications to a literature procedure. 1.4 mL of a solution of ODPAꢀcapped CdSe
nanocrystals (~4 mg/mL) was layered atop 1.4 mL of a Na Cd Se solution in hydrazine
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(~7 mM). The solution was briefly vortexed, and the nanocrystals transferred to the hydrazine
layer. The tolueneꢀrich phase was removed, and the hydrazineꢀrich phase was washed 3 times
with 1.4 mL fresh toluene. Then the nanocrystal solution in hydrazine was filtered through a 0.2
µm PTFE syringe filter. This solution was then precipitated with the addition of acetonitrile
nonsolvent (~1:1 hydrazine:acetonitrile), and the nanocrystals were redispersed in hydrazine.
The above procedure was repeated for (NEt ) Cd Se ꢀcapped CdSe, but the final step required
4 2
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more nonsolvent to precipitate the nanocrystals.
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Preparation of mesostructured CdSe. NꢀeicosaneꢀN,NꢀdihydroxyethylꢀNꢀmethyl ammonium
bromide (EDHEMAB) was synthesized according to a literature procedure, where 1ꢀ
bromoeicosane (7.22 g, 20 mmol) and Nꢀmethyldiethanolamine (2.29 mL, 20 mmol) were
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refluxed in EtOH for 24 hours, and the resulting white solid was washed with cold EtOH and
recrystallized from 1:1 CHCl :EtOAc. EDHEMAB was dried under dynamic vacuum overnight
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and transferred into the glove box. 1g EDHEMAB was dissolved in 12 mL N H at 80 °C to
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form a clear, colorless solution, and 10 mL of 0.025 M Na Cd Se was added dropwise, turning
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the solution cloudy white. This mixture was stirred at 40 °C for three hours. A white solid was
isolated by centrifugation, and washing with excess ethanol and hexane yielded a yellow powder.
This yellow powder could be suspended in 1:1 hexane:ethanol and dropꢀcast for XRD or TEM
analysis.
Synthesis of Na CdTe . 2 mmol Na Te was added to 480 mg CdTe powder (2 mmol) in 4 mL
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hydrazine. After approximately 2 days of stirring, all the solid dissolved, leaving the solution
clear yellow. The solution was filtered with a 0.2 µm PTFE syringe filter, and the solid was
isolated through precipitation with acetonitrile nonsolvent and centrifugation. The solid
Na CdTe could be dissolved in hydrazine or formamide up to concentrations of ~0.5 M.
2
2
Preparation of mesostructured CdTe. 100 mg EDHEMAB was dissolved in 2 mL formamide at
0 °C. 500 µL of a 0.1 M solution of Na CdTe in hydrazine was added dropwise at 80 °C, and
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2
a yellowꢀorange precipitate formed immediately, suspended in a green solution. The temperature
was decreased to room temperature, and the suspension was stirred for an hour. The solid was
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isolated by centrifugation and washed with excess NMF, ethanol, and hexane to yield a redꢀ
orange powder.
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Preparation of gelꢁlike CdSe. 50 µL of 0.5 M CdCl in N H was added to 1 mL of ~0.025 M
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Na Cd Se in N H and briefly vortexed to form a cloudy white suspension. After centrifugation
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for 10 seconds, a white or pale yellow gelꢀlike material was collected, and the supernatant was
discarded to remove excess reactant and NaCl. The gel can be resuspended in varying amounts
of hydrazine to tune its viscosity. To form films, suspensions of the gel were dropꢀcast on
piranhaꢀtreated glass or silicon substrates, dried at room temperature, and annealed at various
temperatures for 30 minutes on a hot plate.
3. Results and Discussion
3
.1. Na Cd Se synthesis and structural characterization. Bulk CdSe powder completely
2
2
3
dissolves in liquid hydrazine at room temperature in the presence of 0.5 molar equivalents of
Na Se. After several days of stirring, the solution becomes colorless, and the
2
chalcogenidometallate product can be isolated by addition of acetonitrile followed by
centrifugation to yield a white powder. Inductively coupled plasma optical emission
spectroscopy (ICPꢀOES) indicates that the product has the empirical formula Na Cd Se . This
2
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3
sodium selenocadmate is soluble up to ~0.05 M (25 mg/mL) in hydrazine.
To characterize this new chalcogenidometallate compound, colorless, platelike crystals
were grown by slow evaporation of hydrazine from a ~0.025 M Na Cd Se solution over the
2
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3
course of two weeks, and the structure was revealed by single crystal Xꢀray diffraction. The
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ꢀ
diffraction results show that this complex crystallizes in the P1 space group with an asymmetric
unit of Na Cd Se · 9.5 N H (Figures 1, S1). The selenocadmate anionic framework has a oneꢀ
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dimensional structure of double chains of distorted edgeꢀsharing [CdSe ] tetrahedra running
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along the cꢀaxis. The structure contains bridging Se anions coordinated by four Cd cations
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with bond distances ranging from 2.7210(7) to 2.7981(7) Å and terminal Se anions bound to
2+
2nꢀ
3 n
two Cd cations through shorter bonds from 2.6075(7) to 2.6280(7) Å. These (Cd Se )
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+
+
2ꢀ
chains are charge balanced by Na cations. The Na either bridges two Se or is bound to a
2
ꢀ
+
single Se with average NaꢀSe bond lengths of ~3 Å; the coordination sphere around Na is
filled with hydrazine in either an octahedral or trigonal bipyramidal configuration. Adjacent
2nꢀ
+
+
(
Cd Se ) chains are connected by Na ꢀhydrazineꢀNa bridges, facilitating mutual alignment of
2 3 n
the chains along the cꢀaxis. Further details on the diffraction data are provided in the Supp.
Information, Table S1.
Figure 1. Single crystal structure for Na Cd Se · 9.5 N H , where grey spheres represent Cd,
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4
yellow is Se, purple is Na, and blue is N: (a) viewed down the cꢀaxis with hydrogen atoms
ꢀ
omitted for clarity with unit cell outlined in red (P1, a = 10.5623(10) Å, b = 13.5662(12) Å, c =
13.6038(11) Å, α = 104.728(3)°, β = 108.183(3)°, γ = 111.179(3)°, Z = 2); (b) a view
highlighting Cd and Se chalcogenidometallate framework to demonstrate its wireꢀlike
morphology.
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The structure of sodium selenocadmate shares aspects of both neutral lowꢀdimensional
metal chalcogenides and anionic chalcogenidometallates. Lowꢀdimensional metal chalcogenides
have been synthesized solvothermally from a metal salt and elemental chalcogen, and the
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resulting structures are one, two, or threeꢀdimensional segments of the bulk lattice.
The extended oneꢀdimensional framework of Na Cd Se is similar to that of the solids
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3,
SnS Mn (N H ) and ZnTe(1,3ꢀpropanediamine) or the hydrazineꢀsoluble species (N H )ZnTe.
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2nꢀ
3 n
However, (Cd Se ) deviates from the bulk structure of cornerꢀsharing [CdSe ] tetrahedra
2
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and demonstrates the bonding motif of edgeꢀsharing tetrahedra. Edgeꢀsharing brings the cations
into closer proximity, yielding a CdꢀCd distance of ~3.34 Å in Na Cd Se compared to a CdꢀCd
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3
distance of 4.3 Å in wurtzite CdSe. This suggests greater covalent character that limits the
repulsion of the Cd cationic centers. The divergence from bulk structure in favor of edgeꢀsharing
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6
tetrahedra is seen in many small molecular chalcogenidometallates (e.g. Sn S , Sn Se , and
2
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2
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ꢀ 28, 30
Ge Se )
and is exhibited by the extended oneꢀdimensional tellurocadmate polyanion
2
6
2nꢀ 36
(CdTe2)n
.
The toxicity and instability of hydrazine make the use of other solvents desirable for
solution processing; however, sodium selenocadmate has not been appreciably soluble in other
solvents tested (DMF, NMF, FA, DMSO, and en). We found that the solubility and versatility of
selenocadmate can be expanded by exchanging the chargeꢀbalancing cation as schematically
shown in Figure 2a. Sodium ions can be replaced with tetraethylammonium by stirring a
+
Na Cd Se solution with an excess of NEt ꢀloaded Amberlystꢀ15 cation exchange resin, and the
2
2
3
4
resulting (NEt ) Cd Se is soluble in hydrazine, 85:15 acetonitrile:hydrazine, Nꢀ
4
2
2
3
methylformamide, and dimethyl sulfoxide. The solubility can be further expanded to nonpolar
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solvents by replacing sodium with didodecyldimethyl ammonium (DDA ). This cation exchange
takes place in a twoꢀphase system where solutions of didodecyldimethyl ammonium bromide
(
DDAB) in toluene or hexane and Na Cd Se in hydrazine are stirred, and the selenocadmate
2 2 3
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transfers to the nonpolar phase. UVꢀvis spectra for cationꢀexchanged selenocadmate in a variety
of polar and nonpolar solvents are given in the Supp. Information (Figure S2).
Figure 2. (a) Cation exchange enables solubility of Na Cd Se in various polar and nonꢀpolar
2
2
3
solvents. (b) UVꢀvis absorption spectra suggest that cation exchange does not change the anion.
EXAFS data from the (c) Cd and (d) Se Kꢀedges demonstrate the reduced coordination number
and increased disorder for Na Cd Se and (NEt ) Cd Se species in solution.
2
2
3
4 2
2
3
The solution structure of the selenocadmate species in hydrazine was further evaluated
with extended xꢀray absorption fine structure (EXAFS) at the Cd and Se Kꢀedges. The details of
a firstꢀshell fit for CdSe powder and Na Cd Se and (NEt ) Cd Se solutions in hydrazine at the
2
2
3
4 2
2
3
Cd and Se edges are given in the Supp. Information (Table S2, Figures S3ꢀ5). The plots of the
Fourierꢀtransformed absorption probability suggest that the selenocadmate structure differs
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significantly from that of bulk CdSe but does not change dramatically with cation exchange
(Figure 2c,d). The reduced amplitude of the Fourierꢀtransformed Xꢀray absorption for
selenocadmate compared to that of bulk CdSe suggests that these species have fewer nearest
neighbor contacts between Cd and Se; though the similar shape of the curves indicates that the
distance between Cd and Se in selenocadmate is consistent with bulk bond lengths. The
coordination numbers for Cd and Se based on the crystal structure of solid Na Cd Se are higher
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3
than those predicted by fitting EXAFS data; this suggests that in solution the selenocadmate ions
exist as smaller fragments that reorganize into long chains as hydrazine is removed. The signalꢀ
toꢀnoise ratio of the EXAFS signal was limited by the low solubility of selenocadmate,
complicating further quantitative analysis of the EXAFS fits.
The electronic structure of selenocadmate in solution was explored with UVꢀvis
spectroscopy. Both sodium and tetraethylammonium selenocadmate in hydrazine show similar
absorption spectra with peaks at 332 nm and 363 nm and a shoulder around 390 nm (Figure 2b).
The close similarity of the spectra indicates that changing the cation does not significantly
change the electronic properties of the selenocadmate anion. Interestingly, the absorption
4
8ꢀ51
frequencies in these spectra are similar to those seen for CdSe clusters.
However, comparison
of extended xꢀray absorption fine structure (EXAFS) data for sodium selenocadmate and CdSe
clusters clearly demonstrates that they are distinct species (Figure S6). This suggests that the UV
absorption peaks may correspond to localized excitations of CdꢀSe bonds and may be generic to
all molecular CdSe species.
3.2. Molecular selenocadmates as capping ligands for colloidal nanomaterials. In previous
studies we have shown that chalcogenidometallates can be effective capping ligands for colloidal
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nanoparticles.
When a solution of Na Cd Se in hydrazine is added to a solution of CdSe
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nanoparticles capped with nꢀoctadecylphosphonate (ODPA) ligands in toluene, the nanoparticle
2nꢀ
ligands exchange from ODPA to (Cd Se3)n and the nanoparticles transfer to the hydrazine
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phase (Figure 3a). Washing the hydrazine with excess toluene removes organics and yields a
solution of inorganicꢀcapped CdSe. Transmission electron microscope images (Figure 3b) show
that the CdSe nanoparticles retain their morphology upon ligand exchange, and infrared (IR)
spectroscopy indicates the organic ligands are completely removed (Figure 3c). Solutions of
2
ꢀ
Cd Se ꢀcapped CdSe in hydrazine can be spinꢀcoated onto substrates and annealed at 300 °C to
2
3
form large grains of polycrystalline CdSe, and fieldꢀeffect transistors constructed from these
2
ꢀ1 ꢀ1 36ꢀ37
films yield electron mobilities of ~300 cm V s .
This remarkable electron mobility
suggests that the compositionꢀmatched ligands not only foster semiconductor grain growth
during annealing but also improve electronic transport across the grain boundaries, serving as a
solder that retains semiconductor performance.
As described above, the use of hydrazine as a solvent limits the conditions under which
the solution can be used. We found that performing ligand exchange with (NEt ) Cd Se rather
4
2
2
3
2
3
ꢀ
than Na Cd Se yields Cd Se ꢀcapped nanoparticles that can be dissolved in the more benign
2
2
3
2
solvent Nꢀmethylformamide (NMF). Ligand exchange was performed as described above with a
solution of (NEt ) Cd Se in hydrazine. The ligandꢀexchanged nanoparticles could be
4
2
2
3
precipitated with an excess of acetonitrile and dissolved in NMF. UVꢀvis measurements of
solution of CdSe nanoparticles before and after ligand exchange indicate that the nanoparticles
2
ꢀ
have retained their size and transferred into the polar solvent. The zeta potential of the Cd Se ꢀ
2
3
2nꢀ
capped CdSe in NMF was approximately ꢀ40 mV, suggesting that (Cd Se3)n is bound to the
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nanocrystals and stabilizes the nanocrystals in solution via charge repulsion. Comparable zeta
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2
potentials have been measured for many chalcogenidometallateꢀcapped nanoparticles.
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Figure 3. (a) Schematic showing the ligand exchange of CdSe nanoparticles from ODPA to
2nꢀ
(
Cd Se ) and the corresponding phase transfer. (b) TEM image of Na Cd Se ꢀcapped CdSe
2 3 n 2 2 3
nanoparticles with an inset showing lattice fringes, scale bar 2 nm. (c) IR shows that signals
corresponding to organic ligands are gone after ligand exchange with Na Cd Se . (d) UVꢀvis of
2
2
3
solutions of ODPAꢀcapped CdSe in toluene and (NEt ) Cd Se ꢀcapped CdSe in NMF. (e) Zeta
4
2
2
3
potential measurement of (NEt ) Cd Se ꢀcapped CdSe in NMF with the peak centered at
4
2
2
3
approximately ꢀ40 mV.
3.3. Templated synthesis of mesostructured IIꢁVI semiconductors. The facile interchange of
chargeꢀbalancing cations in solution allows for templating of chalcogenidocadmate ions using
longꢀchain alkylammonium surfactants to form unprecedented mesoporous CdSe and CdTe
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structures (Figure 4a). Organic surfactants have been previously used as a soft template for
inorganic materials to produce solids with monodisperse mesoscale (2ꢀ50 nm) pores in an
5
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organized array. Surfactantꢀtemplated mesoporous oxides have been widely utilized for their
high surface area and the ease of surface functionalization. Functionalized mesoporous oxides
display high performance in many applications, including heterogeneous catalysts, solar cell
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3ꢀ55
absorber layers, and cathode and anode materials for Li ion batteries.
It is hoped that the
extension of soft templating techniques to nonꢀoxide IIꢀVI semiconductors will yield novel
functional materials. Previous reports have demonstrated the patterning of
4ꢀ
4ꢀ
4ꢀ
chalcogenidometallates (e.g. Sn S , SnTe , and Ge S ) around columnar micelles and
2
6
4
4 10
2
+
4+
2+
2+
crosslinking of the resulting structure with transition metal ions (Pt , Pt , Cd , Zn , or
2
+ 39ꢀ40, 56
Mn ).
These reports suggest that the oligomerization of chalcogenidometallate species
through interaction with metal ions is necessary for the mesostructure formation to be
2nꢀ
3 n
2nꢀ
entropically allowed; however, the extended chains of (Cd Se ) or (CdTe2)n favorably selfꢀ
2
assemble with positively charged micelles without the inclusion of a foreign metal ion. This
work is to our knowledge the first example of surfactantꢀtemplated CdSe and CdTe from
molecular precursors.
An ordered CdSe mesostructure can be formed by coprecipitation of NꢀeicosaneꢀN,Nꢀ
2nꢀ
di(2ꢀhydroxyethyl)ꢀNꢀmethyl ammonium bromide (EDHEMAB) and (Cd Se3)n from
2
hydrazine. When a colorless solution of Na Cd Se in hydrazine is added to EDHEMAB solution
2
2
3
in hydrazine at 80 °C, the resulting mixture immediately turns a cloudy white. After stirring for
three hours at 40 °C, the precipitate is collected by centrifugation and washed with excess
ethanol and hexane to yield a yellow powder.
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Figure 4. (a) Proposed scheme depicting the soft templating of chalcogenidocadmates.
Characterization of mesostructured (b-e) CdSe and (f-g) CdTe. (b) The SAXS pattern for
mesostructured CdSe shows longꢀrange order that can be indexed to an fcc lattice. TEM images
of mesostructured (c) CdSe and (f,g) CdTe show clear areas of metal chalcogenide separated by
surfactantꢀfilled pores. (d) The Raman spectrum for the CdSe mesostructure shows broad peaks
corresponding to CdSe. (e) The diffuse reflectance (converted to absorbance with KublekaꢀMunk
equation) and photoluminescence of a mesostructured CdSe film excited at 3.26 eV indicate a
bandgap of ~2.5 eV.
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The organization of the resulting hybrid inorganicꢀorganic mesostructure was explored
with small angle xꢀray scattering (SAXS) and transmission electron microscopy (TEM). Smallꢀ
angle xꢀray scattering (SAXS) at the Advanced Photon Source at Argonne National Lab was
used to probe the mesoscale order of a large powder sample without selection bias. SAXS
pattern shows a number of resolved peaks and indicates a mesostructure with longꢀrange order of
the pores (Figure 4b). These peaks can be indexed to a faceꢀcentered cubic lattice of spherical
surfactant micelles surrounded by metal chalcogenide, with a possible structure suggested in
Figure 4a. Although hexagonal phases of columnar micelles are most often achieved with
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surfactant templating, the formation of cubic phases has been seen for both metal oxides and
4
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chalcogenides,
and it has been suggested that cubic phases are more desirable because guest
species can access the porous network from all directions. It is possible that the relatively low
reaction temperature or the reaction of selenocadmate with the surfactant and resulting reduction
of templating species yields spherical rather than columnar micelles.
Transmission electron electron microscopy (TEM) was used to further explore the
morphology of this mesostructured CdSe. TEM images show large areas of mesostructure with
surfactantꢀfilled pores of about 2 nm diameter (Figure 4c, S7). However, the TEM images do
not reflect the longꢀrange order suggested by SAXS measurements but rather suggest disordered,
wormholeꢀlike pores. This discrepancy is likely due to the difference in sample preparation for
the two techniques. SAXS measurements were taken on a wet powder suspended in ethanol,
while TEM measurements were taken on a dry powder under high vacuum. The change of
mesostructure organization upon drying is a known phenomenon and is here demonstrated by
5
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our two techniques.
It is likely that ethanol swells the organic template, and order is lost as
the template dries and contracts.
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ICPꢀOES of the mesostructured CdSe powder indicates that the material contains
equimolar amounts of Cd and Se (Table S3). The excess selenium in the Na Cd Se precursor
2
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3
likely reacts with the surfactant to yield CdSe and volatile or soluble byproducts that are
removed during washing. The dealkylation of alkylammonium salts by chalcogenides is well
known and could easily occur at the elevated temperature of the template solution to release a
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soluble organoselenium species. Although ICPꢀOES indicates stoichiometric CdSe, powder xꢀ
ray diffraction shows only broad peaks at higher angles (Figure S8), suggesting that the pore
walls are either amorphous or contain very small crystallites. The Raman spectrum of
ꢀ1
ꢀ1
mesoporous powder exhibits peaks at ~202 cm and ~404 cm corresponding to the longitudinal
optical (LO) phonon mode of CdSe lattice and its first overtone (Figure 4d). For comparison, the
Raman spectrum of Na Cd Se is qualitatively different from that of CdSe (Figure S9), further
2
2
3
confirming that the mesostructured material contains a CdSe phase formed from templated
2nꢀ
(
Cd Se ) ions under our reaction conditions. Thermogravimetric analysis indicates that the
2 3 n
pores are filled with surfactant which is removed at elevated temperature (Figure S10). The
diffuse reflectance spectrum of a film of mesostructured CdSe was measured with an integrating
sphere and converted to absorbance using the KubelkaꢀMunk equation (Figure 4e, S11). This
measurement indicates a bandgap of ~2.5 eV, which is reasonable for strongly quantumꢀconfined
9
CdSe. The photoluminescence of a film of mesostructured CdSe excited at 3.26 eV was
measured, and it agrees with the bandgap measured by diffuse reflectance (Figure 4e).
This technique for creating surfactantꢀtemplated metal chalcogenides can be extended to
form mesostructured CdTe. When solutions of Na CdTe in hydrazine and EDHEMAB in
2
2
formamide are combined at 80 °C, a yellowꢀorange precipitate forms immediately and the
solution turns green due to decomposition of tellurocadmate to yield polytelluride and CdTe.
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The resulting suspension is stirred at room temperature for an hour, and a deep redꢀorange
powder is isolated by centrifugation and washed with ethanol and hexane to remove the excess
surfactant and tellurium. TEM reveals a wellꢀordered CdTe mesostructure templated around
columnar micelles (Figure 4f). Though XRD shows no sharp peaks at high 2θ angles (Figure
S11a), the identity of the metal chalcogenide was confirmed with Raman spectroscopy (Figure
S11b). The lattice fringes of small crystallites can be seen in the highꢀresolution TEM image
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(Figure 4g), likely due to crystallization of amorphous CdTe under the electron beam. A natural
followꢀup to these syntheses is the removal of the templating surfactant or the use of a
59
semiconducting templating agent to create new functional materials.
+
3.4. Gelꢁlike stoichiometric CdSe ꢀ xN H . Unlike the facile replacement of Na by organic
2
4
2+
2nꢀ
3 n
cations, the addition of Cd to (Cd Se ) induces significant changes in the
2
chalcogenidometallate structure and properties, creating a novel gelꢀlike form of CdSe. When
Na Cd Se is combined with CdCl in hydrazine, an offꢀwhite suspension forms (Figure 5a),
2
2
3
2
based on the following equation:
(1)
Brief centrifugation yields a viscous gelꢀlike material and the soluble NaCl product is removed in
the supernatant. Inductively coupled plasma optical emission spectroscopy (ICPꢀOES) confirms
that this gelꢀlike species contains equimolar amounts of Cd and Se and negligible Na. This CdSe
gel is related to a class of chalcogels created by crosslinking chalcogenidometallates (e.g.
4
ꢀ
4ꢀ
[Sn S ] and [Ge S ] ) with transition metal ions; however chalcogels typically use foreign
2
6
4 10
2
+
60ꢀ61
metal ions (e.g. Pt ) to control the crosslinking metathesis and produce a stable gel.
Stoichiometric CdSe gel is attractive as a precursor for semiconductor materials because its only
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possible contaminant is hydrazine, which readily evaporates during annealing to leave a
chemically pure CdSe phase, and its high viscosity is convenient for doctor blading and other
deposition techniques.
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Figure 5. (a) Photographs showing the synthesis of CdSe gel and its high viscosity. (b) The
ꢀ1
Raman spectrum of CdSe gel has a sharp feature at 140 cm , which does not correspond to the
features of Na Cd Se or CdSe. (c) The SEM image of dried CdSe gel has aggregations of thin
2
2
3
sheets. EXAFS data from the (d) Cd and (e) Se Kꢀedges suggests that the CdSe gel has a
structure similar to that of bulk CdSe.
Raman spectroscopy, powder xꢀray diffraction, and EXAFS were used to examine the
ꢀ1
structure of CdSe gel. The Raman spectrum of CdSe gel displays a peak at 140 cm and a broad
ꢀ1
feature below 250 cm (Figure 5b); these features are distinct from those of both bulk CdSe and
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Na Cd Se (Figure S9), indicating different chemical bonding in these compounds. The
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3
appearance of peaks below the energy of the phonon modes of the parent compound has been
4
7
observed for lowꢀdimensional metal chalcogenides, suggesting that the CdSe gel may have
isolated fragments of a tetrahedrally coordinated CdSe lattice.
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The structure of this stoichiometric CdSe gel was explored with powder xꢀray diffraction
(XRD). Removal of the hydrazine solvent leads to structural collapse of the gel material,
yielding a yellow to orange powder that cannot be resuspended in hydrazine, so XRD
measurements were taken on suspensions of the gel in hydrazine held in sealed glass capillaries.
The suspension is largely amorphous and exhibits only weak scattering; however, rings can be
observed in twoꢀdimensional powder patterns (Figure S13ꢀS14). Immediately after synthesis,
the diffraction pattern exhibits one peak at ~8° 2θ angle, but the structure evolves over time,
developing peaks at higher angles as the initial peak wanes in intensity. The structure stabilizes
after 24 hours, exhibiting two diffraction peaks at ~10.5° and ~27° 2θ, and this structure remains
stable in a sealed nitrogen atmosphere (no observed changes up to 60 days).
The poor crystallinity of the CdSe gel limits the quality of attainable powder data, so
direct structure solution followed by Rietveld refinement is not possible. However, we noted
that the peak positions and intensities are similar to those previously seen for layered metal
chalcogenides with bridging diamines. Powder patterns for CdS, CdSe, and CdTe layered
structures with linking amines have the most intense peak at a low angle; 10.6° 2θ is reported for
21
layered CdSe slabs linked by ethylenediamine and 8.6° 2θ is reported for CdSe slabs linked by
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,3ꢀpropanediamine. These intense peaks correspond to the interꢀslab spacing, with larger
distances yielding diffraction at smaller angles. Thus, the observed peaks in CdSe gel at low
angles may suggest that layered slabs of a 2ꢀdimensional CdSe species have formed. The
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evolution of the diffraction peaks with time may represent the formation of different layered
CdSe structures with variable distance between slabs; we expect that hydrazine fills the space
between layers and is expelled as the structure evolves as schematically outlined in Scheme S1.
The SEM images of dried gel show micronꢀsized globules with an internal structure made of
stacked layers (Figure 5c), supporting the above hypothesis. Additionally, thermogravimetric
analysis suggests that the gel composition is approximately 4 CdSe · 3 N H ; two loosely bound
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hydrazine molecules are lost at ~100 °C and a chemically bound hydrazine is lost at ~200 °C to
yield pure CdSe (Figure S16). Further analysis of the gel structure is provided in the Supp.
Information (Figure S14ꢀS15, Table S4).
EXAFS measurements taken on the CdSe gel suspension in hydrazine further support the
proposed structure model. The amplitude of the Fourierꢀtransformed absorption probability of
the gel is higher than that of selenocadmate and more closely mimics bulk CdSe, suggesting
higher coordination numbers for Cd and Se (Figure 5d,e). Fitting of this data (Table S2) yields
2nꢀ
coordination numbers of about 3.5 ± 0.6 for both Cd and Se. The chains of (Cd Se3)n have
2
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crosslinked through addition of Cd , creating many new CdꢀSe bonds and a structure that
approaches that of bulk CdSe. Additionally, the Cd edge is wellꢀmodeled with the inclusion of a
nitrogen shell, suggesting that cadmium is directly coordinated to hydrazine. It is likely that the
edges of the slabs have underꢀcoordinated cadmium that is bound to hydrazine. This direct
coordination of amines to cadmium has been shown to stabilize a variety of lowꢀdimensional
21ꢀ23
metal chalcogenide species.
Hydrazine coordination supports the separation of the chains
and formation of an open gel structure, and the replacement of these CdꢀN bonds with CdꢀSe
likely leads to the gel collapse and the formation of sphalerite or wurtzite CdSe. Indeed, upon
annealing the gel collapses to form CdSe phase as shown schematically in Figure 6a.
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Suspensions of CdSe gel in hydrazine can be dropꢀcast onto hydrophilic glass or silicon
to form films. Upon annealing, the gel loses coordinated hydrazine and decomposes to wurtzite
CdSe; XRD shows that wurtzite CdSe forms as low as 200 °C and grows into larger grains at
elevated temperature (Figure 6b). The chemical and phase purity of the obtained CdSe is
confirmed by Raman spectroscopy (Figure 6c). SEM of a film annealed at 350 °C reveals that
rather than forming a continuous layer, the gel has formed into layered sheets of crystalline CdSe
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(Figure 6d). Previous works demonstrate that bound amines can template lowꢀdimensional metal
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chalcogenide precursors to form rodꢀ or sheetꢀlike crystals,
and the bound hydrazine in the
CdSe gel likely induces the formation of sheetꢀlike crystals rather than threeꢀdimensional grains.
The porous nature of this film may allow for the intercalation of a second material to create
composite materials.
The photoconductivity of an annealed gel film was measured to determine the electronic
performance of the material. Figures 6e and S16 show the enhanced photocurrent through the
gel film when illuminated with a 405 nm laser. The ~60ꢀfold increase in conductivity upon
illumination suggests that the CdSe gel film is not degenerately doped. Moreover, the measured
conductivity across a large channel (~2 mm) suggests that there is a continuous path for
electrons, although the film consists of twoꢀdimensional sheets (Figure 6d).
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Figure 6. (a) Schematic demonstrating the collapse of the layered gel precursor upon annealing
to form sheetꢀlike crystals of CdSe. (b) XRD of CdSe gel films annealed at various temperatures
for 30 minutes under nitrogen with reference peaks shown for wurtzite CdSe. (c) Raman
spectrum of annealed CdSe gel shows peaks characteristic of bulk CdSe. (d) SEM image of
CdSe gel annealed at 350 °C displays sheetꢀlike grains of CdSe. (e) Photoconductivity of an
unwashed CdSe gel film annealed at 350 °C for 30 minutes. (f) Output and (g) transfer
characteristics for an FET with a channel made of an annealed CdSe gel layer. Inset shows a
scheme of the FET device used for the measurements. Aluminum contacts (100 nm thick) were
thermally evaporated onto the film through a shadow mask (channel length: 50 µm, channel
width: 1500 µm).
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Further electronic characterization was performed by measuring a field effect transistor
FET) with an annealed CdSe gel channel. The asꢀsynthesized CdSe gel could not be used to
(
make FETs because the dropꢀcast film is too thick to effectively gate, and the gel does not form a
uniform layer when spinꢀcoated. Instead, we spinꢀcoated a thin layer of Na Cd Se onto a
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heavily doped silicon substrate with a 300 nm SiO gate layer and soaked the resulting film in a
2
CdCl solution in methanol. We thus formed a thin layer of CdSe gel on the substrate and
2
annealed the film at 250 °C for 30 minutes to form polycrystalline CdSe. The electron mobility
of the annealed gel film was measured with a standard topꢀcontact bottomꢀgate FET geometry
(Scheme S2). The output and transfer characteristics (Figures 6f and 6g) indicate a clear gating
2
ꢀ1 ꢀ1
effect with nꢀtype operation and an extracted linear range electron mobility of 2.6 cm V s . The
large negative bias required to deplete the channel indicatives nꢀtype doping in the FET channel.
Similar FET measurements for gel films annealed at higher temperature show increased offꢀ
currents and a further increase in the zeroꢀgate electron concentration in the channel (Figure
S18). To explore the origin of nꢀtype doping, we fabricated FETs with channels made of
hydrazinium selenocadmate that were not treated with the gelꢀforming CdCl solution and thus
2
ꢀ
not containing Cl . The FET transfer curves showed a substantially lower nꢀtype doping level
resulting in FET threshold voltages, V , shifted from about ꢀ60 V (FET operates in the depletion
th
mode) in devices made of CdSe gel to about +40 V (FET operates in the accumulation mode) in
devices made of hydrazinium selenocadmate (Figure S19). Thus, the roomꢀtemperature crossꢀ
linking of selenocadmate by CdCl seems to affect transport in annealed films primarily by
2
ꢀ
altering the doping level, presumably by thermally activated diffusion of Cl ions to
substitutional lattice sites where they act as the donor species. The use of other Cd(II) salts may
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provide a convenient way to control the doping level and carrier mobility in IIꢀVI materials
prepared from the gel precursors.
The values of electron mobility measured for FETs with channels made of annealed CdSe
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gel are comparable to many other nanocrystalline solutionꢀdeposited semiconductors,
but
considerably lower than the mobilities observed for the spinꢀcoated layers of nanoparticles linked
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with selenocadmate ligands.
This difference can be explained by the fact that the fieldꢀeffect
mobility depends primarily on the quality of the interface between the semiconductor channel
and the gate dielectric. Since all charge transport occurs within an accumulation layer only a few
nanometers thick, highꢀmobility FETs use thin continuous semiconductor layers; however, the
morphology of our “fluffy” CdSe gel is not optimal for high FET mobility. Instead, the openꢀ
framework CdSe chains, gels, and mesoporous structures are naturally suitable for building
nanoscale composites, soldering semiconductor grains, and utilizing inorganic semiconductors in
other unprecedented ways.
4. Concluding remarks.
We have demonstrated the synthesis and structure of sodium selenocadmate and its
modification through interaction with various cationic species to yield new forms of IIꢀVI
materials. The oneꢀdimensional selenocadmate polyanion serves as a link between small
9
molecule metal chalcogenide complexes and extended structures, such as nanocrystals and
21ꢀ23
organicꢀinorganic hybrids synthesized with solidꢀstate or solvothermal syntheses.
The
unique balance of reactivity and stabilization afforded by the hydrazine solvent aids in the
creation of an infinite oneꢀdimensional species that retains solubility.
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The coordination chemistry of hydrazine is rich, unusual, and underexplored. In many
respects, N H is an easierꢀtoꢀhandle form of liquid ammonia, showing a strong ability to
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coordinate and solubilize molecular species. N H can be also related to another important
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ligand, 1,2ꢀethylenediamine (en), and its derivatives. However, en typically generates chelated
complexes, while chelating a metal center with N H would require formation of a strained 3ꢀ
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member ring and is therefore highly unlikely. Instead, two lone pairs prefer to form a bridge
between two metal centers, thus favoring the formation of unprecedented coordination
frameworks. When combined with the rich redox chemistry (N H is mostly known as a reducing
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agent but can also behave as an oxidant), we have a powerful agent for the synthesis of lowꢀ
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dimensional semiconductors. Another important benefit of N H is its ability to cleanly
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decompose at relatively low temperatures without leaving behind any carbonꢀcontaining
impurities.
The creation of mesoporous materials by templating inorganic species around organic
templates is wellꢀdeveloped for oxides. However, the creation of templated IIꢀVI
semiconductors is much less explored and often suffers from the inclusion of impurities, such as
foreign metal cations or other linking agents. We have presented a method to form
mesostructured CdSe or CdTe using a surfactant template for oneꢀdimensional
chalcogenidocadmate polyanions. The polymeric nature of the chalcogenidocadmates allows for
favorable coassembly in the presence of a surfactant solution. Further development of these
materials could yield mesoporous IIꢀVI materials with inherent optical and electronic properties
that cannot be realized in unfunctionalized oxide materials. Finally, the gelꢀlike materials
described in this work represent a novel type of precursors for stoichiometric CdSe. This route
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appears to be particularly useful for semiconducting materials, which demand high chemical
purity and structural perfection.
The chemistry presented in this work is not limited to CdSe but can be extended to other
semiconductors in the IIꢀVI family. Stable chalcogenidometallates have been previously
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reported for CdTe,
ZnTe, HgS, HgSe, and HgTe. Our study suggests that cations play
an important role in determining solubility of chalcogenidometallates in polar and nonꢀpolar
solvents. We expect that the techniques demonstrated here for CdSe could be extended to other
systems with the aid of this existing prior art. The mixing and matching of semiconducting
components at the nanoscale represents a powerful strategy toward materials with advanced
physical and chemical properties.
ASSOCIATED CONTENT
Supporting Information
Experimental details and supporting figures. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
dvtalapin@uchicago.edu
Present Addresses
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#
(
J.J.) Department of Energy Engineering, Hanyang University, Seoul 133ꢀ791, Republic of
Korea.
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The authors declare no competing financial interests.
ACKNOWLEDGMENT
The authors thank Danny Haubold for assistance with TEM and Michael A. Boles for help with
editing. This work was supported by the NSF under Awards CHEꢀ1611331 and DMRꢀ1629601,
the Department of Defense (DOD) Air Force Office of Scientific Research under grant number
FA9550ꢀ14ꢀ1ꢀ0367, and by the IIꢀVI Foundation. BL was supported by MICCoM as part of the
Computational Materials Sciences Program funded by the U.S. Department of Energy, Office of
Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Sector 20
facilities at the Advanced Photon Source, and research at these facilities, are supported by the US
Department of Energy ꢀ Basic Energy Sciences, the Canadian Light Source and its funding
partners, and the Advanced Photon Source. This work used resources of the Advanced Photon
Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the
DOE Office of Science by Argonne National Laboratory under Contract No. DEꢀAC02ꢀ
0
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