MATERIALS CHEMISTRY
Composition-matched molecular
“solders” for semiconductors
Dmitriy S. Dolzhnikov,1,2 Hao Zhang,1,2 Jaeyoung Jang,1,2 Jae Sung Son,1,2
Matthew G. Panthani,1,2 Tomohiro Shibata,3,4
Soma Chattopadhyay,3,4 Dmitri V. Talapin1,2,5*
We propose a general strategy to synthesize largely unexplored soluble
chalcogenidometallates of cadmium, lead, and bismuth. These compounds can be used
as “solders” for semiconductors widely used in photovoltaics and thermoelectrics.
The addition of solder helped to bond crystal surfaces and link nano- or mesoscale
particles together. For example, CdSe nanocrystals with Na2Cd2Se3 solder was used
as a soluble precursor for CdSe films with electron mobilities exceeding 300 square
centimeters per volt-second. Cd Te, Pb Te, and Bi2Te3 powders were molded into various
shapes in the presence of a small additive of composition-matched chalcogenidometallate
or chalcogel, thus opening new design spaces for semiconductor technologies.
Two pieces of metal, such as copper wires, canbemechanicallyand electricallyconnected by using soldering, a process in which me- tallic items are joined together by introduc- ing a filler metal (solder) with lower melting
point (1). Soldering and related processes (such
as brazing) are very useful in many areas, from
microelectronics to plumbing. Unlike the case
for metals, there are no established methods for
joining semiconductor pieces under mild conditions without disrupting semiconducting properties at the joint. A “semiconductor solder” could
affect numerous fields, including printable electronics, photovoltaics, and thermoelectrics, because it would add new techniques for device
integration.
The electronic properties of semiconductor
interfaces are much more sensitive to impurities
and structural defects than are metals. Metal-metal contacts always show ohmic behavior,
whereas semiconductor-semiconductor interfaces are more complex. Misalignment of the
Fermi energy levels or trapping charge carriers
at the interface creates a Schottky barrier (2).
An ideal “semiconductor solder” should be constituted of a precursor that, upon mild heat
treatment, forms a semiconducting material
structurally and compositionally matched to
the bonded parts. In this study, we designed
chalcogenidometallate “solders” for technologically important II-VI, IV-VI, and V-VI semiconductors, including CdSe, CdTe, HgxCd1−xTe,
Pb Te, (BixSb1−x)2Te3, and Bi2Te3.
The use of chalcogenidometallates as soluble
precursors for inorganic semiconductors was first
introduced by Mitzi et al., who used the ability
of N2H4 to dissolve metal chalcogenides in the
presence of elemental chalcogens (3), forming
soluble hydrazinium chalcogenidometallates.
These species cleanly decompose back into semi-
conducting metal chalcogenides upon heating.
This simple approach works very well for the
chalcogenides of Cu, Ga, Ge, In, Sb, Sn, and mixed
phases such as Cu(In1−xGax)Se2 and Cu2ZnSn(S,Se)4
(4–6) but showed no success for Cd-, Pb-, and Bi-
chalcogenides (7, 8), which are among the most
widely used binary and ternary semiconductors.
Here, we propose the origin of this limitation and
a general solution, which substantially expands
the list of solution-processed semiconductors.
In the above process, the formation of a soluble chalcogenidometallate begins with the reduction of an elemental chalcogen (Ch = S, Se, or Te)
by N2H4 following Eq. 1 (3, 4)
2nCh + 5N2H4 → 4N2H5+ + 2Chn2− + N2 (1)
The Chn2− reacts with electron-deficient metal
centers at the surface of a solid metal chalcogenide (MxChy), generating soluble MxCh(y+m)2m−
chalcogenidometallate ions:
2nMxChy + 2mChn2− + 5(n − 1)mN2H4 →
2nMxCh(y+m)2m− + (n − 1)mN2 + 4(n − 1)mN2H5+ (2)
The progression of reaction in Eq. 2 is deter-
mined by the balance between the lattice energy
of the metal chalcogenide and the free energy of
formation and solvation of the chalcogenidome-
tallate complex. Reactivity of Chn2− is highest at
n = 1 and decreases as n increases. The reducing
potential of N2H4 in the reaction in Eq. 1 is not
sufficient to bring Se and Te into their most
reactive Ch2− state. X-ray absorption near-edge
spectroscopy (XANES) measurements show that
Eq. 1 generates soluble Se or Te species with an
oxidation state near zero, which is equivalent to
large n (fig. S1) (9). To increase the driving force
for Eq. 2, we used A2Se or A2Te (A = Na, K, and
Cs) instead of elemental chalcogens and found
that for n = 1, Eq. 2 proceeds smoothly for a num-
ber of metal chalcogenides previously considered
unreactive, including CdSe, Cd Te, PbS, PbSe, Pb Te,
Bi2S3, Bi2Se3, Bi2Te3, (BixSb1−x)2Te3, and some others,
which are summarized in (9). These reactions could
also be conveniently carried out in one pot—for ex-
ample, by adding alkali metal hydride to the stoi-
chiometric mixture of Cd Te and Te in N2H4: Cd Te +
Te + 2NaH → Na2Cd Te2 + gaseous products (9).
The structure of K2CdTe2·2N2H4 crystallized
from the reaction mixture is shown in Fig. 1A. It
contains molecular chains built of edge-sharing,
slightly distorted [CdTe4] tetrahedrons with
Te-Cd-Te angles of 99.4° and 119.1° (9). The bonds
between Cd and Te are 2.81 and 2.83 Å in length,
which are similar to that of zinc blende CdTe
(2.81 Å). This structural motif with one-dimensional
(1D) “molecular wires” has apparently not been
reported for chalcogenidocadmates (10, 11). In
the crystal lattice, [CdTe22−]∞ chains are separated with N2H4 molecules, which leads to the
facile solubility of the compound in various solvents, up to 600 mg/mL in N2H4 (Fig. 1B). The
solubility of ditellurometallates could be further
tailored by means of cation exchange (9). For example, Na+ or K+ was exchanged for alkylammonium
cations such as didodecyldimethyl ammonium
(DDA+) or tetraethyl ammonium (NEt4+), providing good solubility of DDA2Cd Te2 in toluene and
of (NEt4)2Cd Te2 in acetonitrile (CH3CN) or N,N-dimethylfomamide (DMF), respectively (Fig. 1B).
Exchange of K+ with hydrazinium cations (9) could
be used to facilitate thermal decomposition of
the resulting hydrazinium salts.
X-ray diffraction and extended x-ray absorption fine structure (EXAFS) studies (figs S2 and
S3 and tables S1 and S2) (9) of ditellurocadmates
suggest that, in solution, [Cd Te22−]∞ chains can
exist in the equilibrium forms outlined in Fig. 1C.
In a strongly coordinating solvent such as N2H4,
the equilibrium shifted toward the [Cd Te(m-Te)]n2n−
structure, where each Cd atom has 3 Te neighbors and Te has on average 1.5 Cd atoms in the
first coordination shell. On the other hand, weakly coordinating solvents such as CH3CN shifted
the equilibrium toward the structure with two
bridging Te atoms per [Cd Te2]2− unit. These variations in coordination environment were reflected
by fully reversible shifts of the absorption bands
in mixtures of N2H4 and CH3CN with various
solvent ratios (fig. S4) (9).
Reacting Na2Cd Te2 with a stoichiometric amount
of CdCl2 in N2H4 formed a white amorphous gel,
further referred to as “Cd Te-gel” (Fig. 1B), which
is an important addition to the chalcogel family
(12). The gel was easily separated from its NaCl
by-product by means of centrifugation and then
redispersed in N-methylformamide (NMF) or
N2H4, forming a stable gel of [Cd Te2]2− polyions
cross-linked with Cd2+. Upon heating above 250°C,
the Cd Te-gel transformed into crystalline Cd Te,
as shown in fig. S5 (9).
[CdTe22−]∞ ions, similar to other chalcogenidometallates (13), were used as capping ligands
for colloidal nanocrystals (NCs). [Cd Te2]2−-capped
CdTe quantum dots showed bright band-edge
photoluminescence (Fig. 1B and fig. S6) (9). The
1Department of Chemistry, University of Chicago, Chicago, IL
60637, USA. 2James Franck Institute, University of Chicago,
Chicago, IL 60637, USA. 3Materials Research Collaborative
Access Team (MRCAT) Sector 10 Advanced Photon Source,
Argonne National Laboratory, Argonne, IL 60439, USA.
4Department of Physics, Advanced Materials Group, Illinois
Institute of Technology, Chicago, IL 60616, USA. 5Center for
Nanoscale Materials, Argonne National Laboratory, Argonne,
IL 60439, USA.
*Corresponding author. E-mail: dvtalapin@uchicago.edu
