1. Two Dimensional
Transition Metal Dichalcogenides
RICARDO VIDRIO, Physics Department, UCSB
RYAN NEED, STEPHEN WILSON, Materials Department, UCSB

2. Two dimensional materials, like graphene and TMDCs,
have a planar structure ideal for flexible electronics.
Graphene
Carbon atoms with hexagonal bonding;
metallic conductivity
MX2
M = transition metal
X = chalcogenide
MM’X4
M = transition metal #1
M’ = transition metal #2
X = chalcogenide
Transition metal dichalcogenides (TMDCs)
Two or more types of atoms with octahedral bonding;
semi-conductivity

3. Drawbacks
All necessary precursors must
be sealed in quartz ampoules
Reactions are slow
Trial and error process
Experimental Parameters
Thot = 1000° C
Tcold = 900 °C
Pressure = 5x10-5 millibar
We used chemical vapor transport reactions
to try to grow NbIrTe4, a ternary TMDC.
This type of reaction is commonly used to grow binary TMDCs (e.g. WTe2, NbSe2)
Initial powders
Intermediates formed by
powder reacting with a
halogen transport agent Crystals
(hopefully)

4. Our CVT reactions resulted either in
reacted powder or small crystals.
powder
crystals
If we got powder, we would:
(1) Remove it from the ampoule
(2) Grind it with a mortar and pestle
(3) Use x-ray diffraction to determine the
phases present and their relative amounts
If we got crystals, we would:
(1) Remove them from the ampoule carefully
(2) Use electron microscopy to determine the
habits and morphology
(3) Use energy-dispersive x-ray spectroscopy
to determine the chemical composition

5. XRD on our powder showed inconsistency
between the nine batches we attempted.
XRD patterns from four select batches. The last two
patterns are from nominally identical batches.
Constructive interference of x-rays diffracted from a series
of parallel planes within a crystal lattice gives rise to peaks.

6. Chemical and electrical analysis on our crystals
revealed them to be pure tellurium.
Tellurium
NbIrTe4

7. Previous chemical vapor transport reaction studies have been primarily focused on
binary systems. These have fewer possible products.
The added complexity of a third element
lead to these inconsistent and undesired results.
The addition of this third element, M’, into our reaction greatly increases the number
of possible products and inhibits formation of pure MM’X4.
W(s) + 2Br2(g) ⇌ WBr4(g)
Te(s) ⇌ Te2(g)
Hot end (vaporization)
WBr4(g) + Te2(g) ⇌ WTe2(s) + 2Br2(g)
Cold end (deposition)
Nb(s) + 2Br2(g) ⇌ NbBr4(g)
Te(s) ⇌ Te2(g)
Hot end (vaporization)
NbBr4(g) + Te2(g) ⇌ NbTe2(s) + 2Br2(g)
Cold end (deposition)
NbBr4(g) + 2Te2(g) ⇌ NbTe4(s) + 2Br2(g)
Ir(s) + 2Br2(g) ⇌ IrBr4(g)
3IrBr4(g) + 4Te2(g) ⇌ Ir3Te8(s) + 6Br2(g)
NbBr4(g) + IrBr4(g) + 2Te2(g) ⇌ NbIrTe4(s) + 4Br2(g)

8. We went back to the drawing board
and began trying a new form of growth reaction.
Tellurium
Niobium
Iridium We placed tellurium in the bottom of a sealed ampoule
in hopes that when heated to 1000°C,
the other powders would mix into the molten tellurium.
Tellurium has a considerably lower melting point
(450°C) than either niobium or iridium (~2400°C).
In fact, the powders did not mix well, likely because of
the confining geometry of the ampoule.
Instead, some small amount of niobium and iridium dissolved
at the interface of this molten tellurium and crystal grew.

9. Results from our first batch using this layering
revealed mm-sized crystals of NbIrTe4.
Crystals had flat, plate-like geometry
that flaked easily when handled.
EDS revealed a 1.2:1:4 ratio
strongly indicating we had NbIrTe4.

10. • Repeat and hopefully replicate growth of NbIrTe4 crystals.
• Determine whether the layering of niobium and iridium is necessary?
Is there a optimum order for layering?
Would mixing those element powders be better?
• Optimize growth conditions (i.e. soak temperature, ramp rates)
for NbIrTe4 crystal formation.
• Once growth of NbIrTe4 is optimized, we will move on to previously
unstudied forms of two dimensional transition metal dichalcogenides,
such as ZrIrTe4, that may harbor unique properties.
Moving forward, we will…

11. This project was partially supported by the LSAMP program of the
National Science Foundation under Award no. DMR-1102531 and by the MRSEC
Program of the National Science Foundation under Award No. DMR- 1121053
I also acknowledge the contribution that Ryan Need has placed forward on
this project. His mentorship and patience proved invaluable throughout the
entire summer. I also want to thank Prof. Stephen Wilson for allowing me to
research in his lab this summer.
Acknowledgements and references:
1. http://www.nature.com/nnano/journal/v7/n11/images/nnano.2012.205-i1.jpg
2. Schmidt, Binnewies, Glaum, Schmidt. Adv. Topics on Cryst. Growth. InTech, (2013) Chp. 9, 227-305.
3. Brown, B. E. The crystal structures of WTe2 and high-temperature MoTe2, Acta. Cryst. 20 (1966) 268.
4. http://www.ualberta.ca/~pogosyan/teaching/PHYS_130/FALL_2010/lectures/lect36/lecture36.html
5. http://chemwiki.ucdavis.edu/Analytical_Chemistry/Instrumental_Analysis/Diffraction/Powder_X-
ray_Diffraction
6. http://www.microscopy.ethz.ch/bragg.htm

12. • Repeat and hopefully replicate growth of NbIrTe4 crystals.
• Determine whether the layering of niobium and iridium is necessary?
Is there a optimum order for layering?
Would mixing those element powders be better?
• Optimize growth conditions (i.e. soak temperature, ramp rates)
for NbIrTe4 crystal formation.
• Once growth of NbIrTe4 is optimized, we will move on to previously
unstudied forms of two dimensional transition metal dichalcogenides,
such as ZrIrTe4, that may harbor unique properties.
Moving forward, we will…

13. Percent yield of powder NbIrTe4 present
among all of our ampoules
Batch Number Powder Ratio Percent NbIrTe4 Transport Agent
2 1:1:4.3 97.7 Br2
3 1:1:4 93.4 none
4 1:1:8 0 none
5 1:1:4 0 none
6 1:1:4 0 I2
7 1:1:4.3 0 none
8 1:1:4.3 5.4 Br2
9 1:1:4.3 73.7 I2

14. Amounts of powder used in experimentation
All mass amounts are in mg
Element Mass with excess
tellurium (1:1:4.3)
Mass without excess
tellurium (1:1:4)
Mass with excess
tellurium (1:1:8)
niobium (Nb) 111.4 116.8 782
iridium (Ir) 230.5 241.6 147
tellurium (Te) 658.0 641.6 71.1