2DCC-MIP Bulk Single Crystal Growth of Novel Quantum Materials


Kevin Dressler: Good afternoon everyone and
welcome to the most recent 2DCC webinar. Today we have Dr. Zhiqiang Mao, who recently
moved to Penn State from Tulane. He is in the department of Physics, and he
will be talking about Bulk Single Crystal Growth Today. Dr. Zhiqiang Mao: Thank you for the introduction. So it is my pleasure to have the opportunity
to give this presentation. So today I am going to talk about bulk single
crystals of novel quantum materials Here is an outline of my talk. First I will give brief instruction on the
concept of crystallization, and then I introduce several crystal growth methods including solution
growth, chemical vapor transport, Bridgeman and floating zone. And so these methods- these three, solution,
chemical vapor transport, and floating zone are heavily used in my research. Bridgeman is used by 2DCC right now. And then I will talk about strategy for crystal
growth of materials that result known phase diagram. That is usually very challenging so I will
talk about it. And I will make brief instruction to my crystal
growth efforts of novel quantum materials. Okay so single crystal refers to, you know,
the material with consistent elements arranged in a periodic pattern arranged along the x,
y, and z direction. As shown in these diagrams. This one example, this is diamond meaning
carbon islands arranged periodically along all three directions And this contrasts with
polycrystalline material powder which contains a number of micrograins and each grain is
a single crystal, but the different grains are oriented in different directions, so that
type of material is called polycrystalline. Single crystal can be grown from polycrystalline
material either using a solution method or a melt growth method. We will talk about that later. Single crystal exist in nature, can be grown
in the lab, but also you can find them in nature. So here are some examples. Quartz, ruby, which is aluminum oxide doped
by Chromium, or blue sapphire, all these you can find in nature- which is also alumina
with different impurities. So these natural crystals are good for making
jewelries, and diamond which is very expensive. So single crystals are also very important
in industry. They have a wide range of applications in
industry including silicon chips, the laser, solar panels- the solar panel can use both
polycrystalline Si or single crystalline silicon, but the single crystal silicon panel has much
higher efficiency than polycrystalline panel. So that is why single crystal silicon production
very important for industry. And also I want to point out that single crystal
samples are critically important for revealing the underlying physics of quantum materials. The research of quantum materials is pulled
from condensed matter physics and materials science, so getting single crystal samples
for these studies are critically important. Some example quantum material include unconventional
superconductors, strongly correlated materials, magnetic materials, the low dimensional materials,
topological materials, and many others. Many examples. So getting single crystals is very important
for research. So next I am going to talk about the crystal
growth method. So in general, crystal can be grown from a
melt, or solution, or vapor phase, or even solid phase, so in general we divide the techniques
into four different kinds as listed here, and I can also regroup a different way in
terms of growth temperature. So high temperature growth you will usually
use these methods. These are Czochralski, Bridgeman, Floating-Zone,
high temperature usually 1200 degrees C. Medium temperature methods include fluxes, vapor
phase transfer, hydrothermal, electrochemical from melt, sublimation, and usually the growth
temperature for these techniques us below 1200. Low temperature methods are usually below
100. So in this presentation, I am going to introduce
these four methods: flux CVT, floating-zone, and Bridgeman, which are heavily used in my
research and also in 2DCC. So first I want to talk about the driving
force for crystallization in solution growth. Solution growth is one of the major methods
used in synthesizing quantum materials. So in solution growth, of course, to get single
crystals, you need to have crystallization. That means that the liquid phase needs to
nucleate and then gradually develop into single crystal. So usually it undergoes two stages in crystallization. So first stage is the formation of three-dimensional
microscopic nuclei. Of course, first of all you need to prepare
a solution. To prepare the solution you need to dissolve
the compound in a solvent, and all of it you call solution, so you need to find an appropriate
solvent to dissolve the desired compound, polycrystalline material. And then you want to- nucleation happens in
this solution. This can be done by several approaches which
I discuss in a few minutes. In the second stage when nucleation occurs,
they need to grow to large single crystal visible to your eyes, so two stage. Then the question is how does crystal nucleation
occur, the whole making it happen, and how it happens we need to understand that what
is the principle they follow for nucleation. So to understand that we first need to understand
these two concepts. One is the solubility as shown in this diagram. So solubility, when we dissolve a material
in a solvent, it is dissolved, and the silicon concentration solute has a certain concentration. So this curve refers to the equilibrium solute
concentration. So this curve is also called the solubility
curve, so this is at equilibrium now. But we can add more solute to the solution,
so in this case and then the concentration at finite temperature would be higher than
the equilibrium concentration. When this happens we call the solution is
saturated. When you continue to add more solute the concentration
will further increase and then we call supersaturated. So this curve is called supersolubility line. So that refers to the limiting value of solute
concentration. So when you have reached this concentration
then nucleation will happen instantaneously, so precipitation will start to take place
above this curve, The crystallization requires a supersaturated solution. So in this region the nucleation will never
happen. In region below this solid curve you do not
expect any nucleation. [9:10] And then you may say we should go to
this region, we can get to nucleation very quickly, that is true, so if you reduce temperature
from A quickly into this region, you will reach the supersaturated region, so in that
case the solute will start to nucleate very quickly, and very fast, but actually that
does not favor crystal growth. You have too many nucleations, they happen
very fast, and also that process introduces a lot of defects so crystals cannot be made
large. You can get micro crystals in this region. [10:00, 2h] So this is the region for crystal
growth. That is called metastable nucleation zone. So here it is not supersaturated, but it is
saturated so that in solution the growth process- So what is really important to us is this
metastable nucleation regime. So then we wonder how we get to this metastable
nucleation. There are three approaches, one is cooling
the solution. Lets say we start from A, point A, and then
we gradually decrease temperature so when it passes point B, the solution which is saturated
but not supersaturated will enter metastable nucleation zone. So this is one approach, you can move from
undersaturated so to metastable zone by cooling the solution. The other approach is solvent evaporation,
so at finite temperature if you evaporate the solution, the solvent, then the concentration
will increase, right? If you start from here and evaporate the solution
the concentration will increase. It will enter first metastable and then supersaturation,
right? So that is the second approach. Or you can do combination, cooling and the
evaporation. So the crystallizing speed actually is dependent
on the supersaturation. Supersaturation means the difference of solute
concentration, os if we start from A and decrease temperature to this point, to t1, from t1
to t2, so the concentrations then are different, right, between c2 and c2. So crystallization speed actually is proportional
to that supersaturation. So here n represents the number of nuclei
formed per unit of solvent volume. So from the above discussion now we can see
that the driving force of crystallization is supersaturation. That is the key word, alright? So we need to have supersaturation to have
nucleation and crystal growth. We do not want to get into this region, supersaturated
unstable region. But you should get into the metastable saturated
region. And the supersaturation can be measured by
this concentration difference, as I just explained. And then you all know why I did- so this is
an experimental observation so what is the physics behind this observation. So here is the physics behind. So actually this nucleation and growth of
crystals is governed by the change of Gibbs free energy, so here delta-G represents the
change of the Gibbs energy, before the nucleation and before reaction, and this is a pretty
major final product. That means that with nucleation happening,
total free energy of the system decreases. In solution growth, the decrease in the free
energy is proportional to the supersaturation, which is the difference of what solute concentration. In melt growth the principle is different. In melt growth we do not use a solution, you
just melt the source material, which could be either polycrystalline material or element. So you do not use any solution. So in that case, the decrease in Gibbs energy
will be proportional to delta-T. Delta-T is the difference between the temperature of
melt and the melting temperature of the crystal. So the principles between these two growth
methods are different. [14:52]
So next I will be taking about the nucleation process. Nucleation takes placed in two different ways. One is homogeneous nucleation. The other is heterogeneous nucleation. So now let me tell you what they are- what
is the homogeneous nucleation. So homogeneous nucleation usually takes place
spontaneously, randomly, and requires supercooling of the solution, or you could say medium. So if the solution has no impurities at all,
so it is very pure, and then the system will undergo supercooling. So let me explain what that means, supercooling. So for a given solution, and then we decrease
the temperature, and if the solution is very pure and in a large container, even in the
metastable nucleation zone it doesnt nucleate. It will continue to remain in a solution without
any nucleation, down to a point C. Beyond C, as I said, nucleation will happen spontaneously. So this process from B to C is supercooling. So for pure solution, that could happen. In that case, it will be very difficult to
get large single crystal because nucleation will only happen in this region. In this region we will not be able to get
large single crystal. So here I want to give you one example of
supercooling: supercooling of pure water. So now lets consider pure water, and then
we reduce temperature, so we try to get the ice crystals. So in general for tap water you reduce temperature
you will get ice at zero degrees C. However, if it is pure water, without any impurities,
you can not get ice at zero degrees C. Instead, you will get ice crystals when temperature
is reduced down to -48 degrees C. The reason for that is the essence of supercooling. So it remains liquid water down to this temperature. And then we further decrease temperature,
it starts to nucleate, forming ice crystals, and then you can further decrease temperature
and then when the ice crystal forms, the temperature will go back to zero degrees C. Of course
if you further decrease temperature the — of ice can also decrease. So that is the homogeneous nucleation process. The other nucleation process is the heterogeneous
nucleation, and this happens when the solution has impurities, like dust, or also the phase
boundary, or even a surface or container can serve as the nucleation site. So when you have that, nucleation can happen
more easily. Of course, you know, when you do solution
growth, the solution has to be put into the container, it always has a boundary. In some cases, a boundary could serve as a
nucleation site. But if you have an impurity, they can become
preferential nucleation site, so when you have those- when you reduce the temperature
and enter the metastable saturation regime, so you have some nucleation around the impurities,
and then when you further decrease temperature, those nucleated particles will continue to
grow and become larger single crystals. So those are the two nucleation processes. So in reality, this nucleation process, the
heterogeneous, takes place much more often than homogeneous. Here is one example of heterogeneous nucleation. So this is soda water, soda water is carbonated
water, right? To facilitate nucleation, you just dip your
finger into the soda water, and you will find a lot of carbon dioxide molecules will nucleate
on your finger. So thats what it means to have heterogeneous
nucleation. So here your one finger is dipped in the water
is an interface between the water and the surface of your finger, thats a nucleation
site so that the carbon dioxide molecules can nucleate. The next concept I want to talk about is the
nucleation barrier. So when you want to grow crystals, the nucleating
particle has to overcome the nucleation barrier. So now we may use this curve to explain what
it means. It is basically the difference of Gibbs energy:
between interfacial Gibbs energy and the volume free energy. Meaning the free energy of nucleating material,
the nucleus — energy diagram. Because when nucleus forms you have a surface,
so you have an interface between the new particle and the solution. So that bringing interfacial energy, increases
the energy. But when nucleation happens also this process
reduces energy in the bulk, right? So they compete, right? So if you add these two curves together then
they become this green curve. So this delta-G-star is called nucleation
barrier. So when these two energies compete, this is
much greater than this, and then you will be in this green region. So in the green region, the small particles
can form, when they compete they can form, but they are dissolved back to liquid very
quickly, so you cannot keep a stable particle long in the green zone. So thermodynamically it is unstable in this
zone. So in general in this zone, homogeneous nucleation
happens in this zone. And then when you further decrease temperature,
in some case you can enter, in other cases you cannot enter. In some case you can enter the yellow region. In this yellow region, the particle has already
overcome that barrier, the delta-G-star. If the particle overcomes that, they can further
grow, and if you further decrease the temperature you enter the red zone. In the red zone, this energy has overcome
that interfacial energy, so that favors further growth of the particle. And you can get the large single crystal. So some cases you are able to get large single
crystal at the end of the growth. That means you overcame that barrier, the
nucleation barrier. You are here now. But in other cases you can not ger single
crystal meaning you did not pass through that nucleation barrier. It is important to bulk crystal growth that
you have to overcome that. But then you wonder how can I overcome that. How can I pass that barrier. So this depends on what solvent you use. So selecting appropriate solvent is critical
for solution growth. If solvent is not the right solvent, that
could be high. It would be difficult to grow crystals. So the finding of appropriate solvent, or
also called flux. In some cases, especially for growth of new
materials, it is difficult. So next I will talk about the criteria for
selecting the appropriate solvent, or flux. So this is just general criteria. First, the flux should exhibit sufficient
solubility of the compound. So when you select a solvent you need to ensure
that your material � material or element can be dissolved in the solvent. If it cannot be dissolved then you will not
be able to grow crystal. And second, the solubility should show sufficient
change with temperature. So when we do cooling, slow cooling of the
crystal, you want the change of the concentration to be large. So in cooling from A to E, you want the large
change in concentration between E and A. But if the curve has small slope than the change
will be small but as I just said, growth speed is proportional to this difference of concentration. So the large slope of this curve is important. So meaning it should have as strong temperature
dependence. Third, the flux material should have a lower
melting point. If it has a very high melting point above
1200 degrees C, you cannot form liquid so you cannot form solution to grow crystal. So usually we use material that has melting
point below 1000 typically. Of course the material needs to have a low
vapor pressure. If it has a high vapor pressure, when you
do growth at high temperature, the solution just vaporizes. If your material is sealed in a quartz tube,
the high pressure will break your quartz tube. So you need to ensure that your vapor pressure
is low. Also low viscosity solutions. If it has a high viscosity, that disfavors
the formation of more high quality materials, because high viscosity could cause roughness
between the surface, between interface, between the liquid and the nucleating particle. So that interface becomes very rough, and
they are all pretty low defects, and also even preventing further growth of crystal. Of course if you want it to be low toxicity.. And also you should not corrode the crucible
and the furnace. So you have to consider that as well. And last, the solvent flux needs to be easily
separated from crystal. In general we have several methods to separate
flux from crystal at the end of the growth, depending on what solvent you used. Here I have listed commonly-used fluxes for
high-temperature solution growth. So this includes the salts. Many salts have a lower melting point, shown
here, all below 900. Some oxides can be used as flux, such as borate,
and bismuth oxide. They also have a lower melting point. Or hydroxides, or even low melting point metals
like tin, gallium, antimony, and bismuth. So of course, if the source is used as the
flux, its easy to separate from growth. You just put your product into water. The salt could be dissolved in water. The crystal will not so you can take the crystal
out from the water. That is the easy way. But if a metal is used as flux, it cannot
be dissolved in water. So then you have to use a centrifuge to separate
the excessive flux metal out. The centrifuge that means that first we need
to put steel bolts into a furnace heated to high temperature. Of course we need to make sure that the temperature
is not higher than the melting point of our crystal, otherwise your crystal is melted. So you need to find a temperature that is
below the melting point of the crystal. So heat it for 2 minutes, and then you quickly
take it out of the furnace and you put it in a centrifuge for one minute- two minutes,
and then the excessive flux will be separate from the crystal. In some case you use some acid or special
chemical solution to dissolve the flux. Also I will briefly mention the self flux. Self flux, meaning in growth you do not have
to add any solvent. If the starting materials include an element
which has lower melting point, that element itself can serve as a flux. At the same time it can also react with other
elements. So its like melt growth. So in that case its easy cut we cannot be
always lucky. Some material can be grown that way, you do
not have to find any solvent. You just mix the chemicals right, of course
according to the mole ratios, depending on what material you grow. And then you heat it up to high temperature
and melt all elements and at the end they will form crystals. And also I want to make some comments on choice
of a crucible. So first a crucible needs to have a high melting
point, otherwise, I heat it to high temperatures to melt, so typically we use these types of
crucible: alumina crucible, zirconium oxide, platinum, tantalum, and niobium. All these have very high temperature. We often use this, and these two are very
expensive, so I never use this because it is. But
in the future we are going to use this. 2DCC has purchased equipment which allow us
to — in tantalum so we are going to use that in the future. Ok so next I want to briefly talk about the
process of crystal growth, crystal growth techniques. So depending on the methods of achieving supersaturation
in the solution, the technique of crystal growth from high-temperature solution can
be grouped into three types. As I said earlier, supersaturation can be
achieved by either cooling, that method is called slow cooling method, or could be achieved
by evaporation of solvent. That is the solvent evaporation method. Or could be achieved by setting up a temperature
gradient in the solution. So here I just want to make a brief comment
on slow cooling. So this shows you how we grew crystals through
slow cooling. So if you know the melting point of the desired
compound then it is straight forward. It is a lot easier. You either use element or polycrystalline
material as the source material. If your polycrystalline material is made and
you know its melting point, you just increase temperature 50 Kelvin above its melting point,
right? So by the melting point, the polycrystalline
material will melt, but the temperature needs to be higher to get more homogeneous of melt. So 50 Kelvin is usually high enough for getting
homogeneous solution. And then you do very slow cooling, with cooling
rate usually between 1 and 3 degrees C per hour. That what would usually be done. So the slow cooling ensures we will enter
this metastable nucleation zone. If you do very quick cooling, like 50 degrees
C per hour, or 100 degrees, you will jump into this region, right? And in that case you just have homogeneous
nucleation, and from that you cannot get large single crystals. For this method, it is useful for growing
material which is stable only in very narrow temperature range. So if this is the case, you cannot use this
method of slow cooling. You need to evaporate the solvent to increase
the concentration to reach supersaturation. But overall, the control of the evaporating
solvent is not that easy. So I never use that in my work, but this can
be done. Next I want to very briefly introduce chemical
transport method. So here I just assumed we make a binary compound
using chemical vapor transport. So to do that, this intermetallic compound,
we have to seal that material in a quartz tube. We put the source material, it could be element
or could be a polycrystalline material. We put it in a hot zone of double zone furnace,
so it has a temperature gradient. This is the hot zone, that is cold zone. And also you need to add a transport agent,
here which is represented by air, that is the transport agent. So when A or B, both need to have high vapor
pressure, when they are vaporized, could react with transport agent, forming a new molecule
A/B, and then they are transported to the lower end. At the lower end, so this molecule will react
with the other elements, forming the desired compound at the cold end, as shown in this
diagram. So the principles of this method are pretty
simple. Next I want to talk about the Bridgeman method,
which is used by 2DCC. I have never used it, but I will use it in
the future. So this slide summarizes the growth of crystals
using Bridgeman method. So this method, as I said, is used by 2DCC. Sam Lee is the expert of crystal growth using
this method. So now let me, very briefly, introduce the
principle of this growth method. So first of all, this is melt growth. It is not growth from solution. It is melt growth. So this diagram shows the process. This curve shows the temperature profiles
of the furnace. Furnaces have three temperature zones that
can produce these profiles. So first step, we seal the source material
in quartz tube, and then we increase temperature to allow reaction to happen. Temperature needs to be higher to get homogeneous
liquid phase. And then we translate this quartz tube to
cold zone, as shown here, so when liquid cools down it will crystallize. At the end, we need to anneal. So when solidification finished, then we need
to anneal the product and the certain temperature will depend on the material you grow. So this method is very effective in growing
most topological insulators, as shown here. These are beautiful crystals grown by Sam
Lee, and these crystals are grown by Dr. –. So the quality of these crystals is pretty high,
as shown here in the x-ray diffraction. So finally, I want to talk about the floating
zone growth technique. So this technique was designed probably 1950
for silicon crystal growth. So, you know, the other major method for silicon
growth is the Czochralski method. However, this method for the silicon has an
advantage, and that advantage is the impurity concentration in silicon crystal could be
two to three orders of magnitude smaller than crystals grown by this method. So in this method, the heating source is the
inductor,, which is just like a single coil and then you apply high frequencies to generate
heat and that can melt the material. So the crystals can be very very big and very
pure. And then in 1960s the optical floating zone
was developed by these two people. In this type of floating zone, the heat source
is not an inductor. It is a lamp. It could be halogen lamp, or could be xenon
lamp, that is more powerful, or it could be laser. So my floating zone furnace uses a halogen
lamp. So here is a schematic of the floating zone. So the lamps are mounted on the focusing point,
and this point is the shared focusing point. So as you can tell, the light reflected from
internal surface can refocus at this point. So the temperature could reach, for my floating
zone furnace, around 2000 degrees C. And in some cases, if this is used, it can go up
to 3000, which I do not have. And to make crystal, first off you need to
make a rod, a material rod, seed rod first. So you use polycrystalline material powder
to make a rod. Because of
the length of time I will not introduce the details of making a rod. And also we need to make a seed rod using
the same material. A seed could be a polycrystalline rod or could
be single crystal. But using single crystal has its advantages. It could quickly make nucleation happen, and
crystal growth takes place very quickly, and also you can control the orientation of the
crystal. So an advantage is that this method does not
use a crucible, so that you do not get contamination from the crucible. That is why the crystals grown by floating
zone method can be very pure. The crystals could be very large, and large
crystals are very useful for neutron scattering for physics studies, and also, of course,
for industry applications. And also, see, in this technique, the thermal
gradient at the crystallization front is very high. Here is the crystallizing front, the thermal
gradient is very high. That decreases the danger of supercooling. So we do not want supercooling, as I just
explained, so that gives us the best growth. And also this type of furnace has a CCD camera
behind the furnace, which allows you to observe the crystal growth and also gain more control. So this type of growth method has gained popularity
for fast growth of oxide, which are difficult to be grown using other methods in past decades. So oxide growth is mostly based on this method,
optical floating zone growth. So I will tell a little bit about the growth
process. When you increase the temperature, the melting
zone, you get some liquid, right, from the seed rod, and then you connect them with the
seed rod. At the beginning, no nucleation happens. As time goes on, you get some micrograins,
and they will continue to grow the small crystals. But we do not want to get crystal with many
many domains, so you have to adjust the growth conditions. Many parameters could be adjusted, so one
domain will dominate. So the growth speed of that domain should
be higher than any other domain, so eventually if you continue you will get larger single
crystal. So the quality of the grown crystal depends
on the growth stability. So the key point for floating zone crystal
growth is to keep stable growth. So stable growth depends on many factors,
as listed here, depending on the shape of the starting rod. The rod needs to be straight and uniform. So sometimes it is very difficult to make
such a rod; a straight and uniform rod. And you have to align these two rods in excellent
shape. You have to align their axes along central
axis. If it is misaligned, that could cause problems. We will see this. When they are connected you will see this. So when this happens you cannot get good crystals. And this � depending on heating power can
be controlled by the voltage. If this — is lower, this is what will happen. Melting zone becomes very thin and it will
just become separate between feed rod and seed rod. And the viscosity needs to be moderate. It cannot be very high. If it is very high, as I said, it will produce
a lot of feedback. And the ideal shape is T-shape. That is ideal shape for crystal growth. And sometimes if the material has high vapor
pressure, you need to apply pressure to suppress the evaporation. So those factors could be optimized to maintain
stable growth. So this is my floating zone furnace moved
from Tulane. So this has two mirrors, maximum temperature
2150 degrees C. What is unique about this furnace is it has equipped a cold trap. You cannot see it here but her it has cooling
water running to the quartz tube. So that can attract the vaporized material. Otherwise the quartz tube can be contaminated
very quickly. So this is a custom design. This is used for oxide growth. It will all be installed in my lab here. And also we have recently purchased a Quantum
Design floating zone furnace. This is new, it is already delivered and will
be installed soon. It also has two mirrors and a cold trap, but
temperature is not that high. It is less than 2000. So this will be used for growing intermetallic
compounds. Okay so this is my new lab. It is still under renovation. But the — will not be put in this lab. Will be in a separate lab. So this lab will be equipped with many furnaces,
including a multiple zone furnace, crucible furnace, Muffle furnace, and I also plan to
buy a arc melting furnace for Cz growth. And the 2DCC also has one Bridgeman furnace
which will be put here. And I will also get the tantalum tube sealer. Dressler: – We have a person streaming the
webinar may have a question. The question was, can you name some methods
how to cool solutions slowly? I think they were curious about that. Mao: So, you know, the cooling rate can be
controlled, right? So any furnace which has the capability of
programming, and then you can control the cooling rate. Yeah that can be done through the program. Dressler: Okay, does it matter per machine,
so for example if you have a floating zone versus Bridgeman with different techniques? Mao: Oh so for solution growth that can be
done by programming. For floating zone growth, the growth speed
is controllable, so the range from 1 mm/hr up to 60 mm/hr, so you can tune growth speed. When growth speed is slow, you have two consequences. One is the vaporization rate is high, but
the cooling rate is lower, so can be controlled by the growth. Ok if you want to finish with five minutes
I will skip these slides. So at the end, so I will skip this slide. So this slide I basically will tell you, so
if you want to grow a new material, a new complex compound with unknown phase diagram,
how would you do that? What method would you use? So this diagram shows you what strategy you
need to follow to identify appropriate growth method. So I am not going to discuss it because it
takes time, but you can look at my slides. It explains it here. So these are examples. So in the last few minutes, I want to make
a very brief introduction to my research. So my research interest is to discover and
understand new quantum materials through synthesis and the characterization. So this summarizes what I have been doing,
and what I will do at Penn State. So basically crystal growth, material characterization,
property and structural characterization, and material physics. We also study material physics, including
superconductivity, magnetism, quantum transport, and topological properties. So we have been using solution method, chemical
vapor transport, and the floating zone to grow crystals of many different kinds of quantum
materials, this including superconductors, topological materials, Dirac/Weyl semimetals,
and low-dimensional materials. In recent few years, we have made a lot of
effort in growing this type of material, layered magnetic materials with a van der Waals gap. And so all of these show magnetic order or
even — order. And then they have van der Waals gap in order
so we can exfoliate to achieve 2D materials. So we have made a lot of new 2D materials,
as shown here. So these are old materials, not studied before. Synthesized, but not studied. So we made single crystals of these materials
using the methods I just introduced. So we have shown all of these 2D materials,
so very interesting properties. I am not going to tell you the details of
the method. And also I will tell you that oxide should
be grown using floating zone. So we have been growing a lot of oxides, mostly
ruthenate and magnetite. So I have done a lot of study on these materials. Its my background. These are strongly correlated materials and
� work on that. And so finally, I also will tell you now my
research has been focused on these four conductors superconductors and exotic phenomenon for
correlated materials, topological and the 2D materials. So here are some representative publications. At the end I want to acknowledge my collaborators,
my former group members they have already graduated. And Yanglin Zhu is not graduated, she is already
here, and will be graduating this year. So now I am doing my new group at Penn State. � with me, and also I recruited several
new students. And the work I have been doing is supported
by NSF and DOE, and recently, of course, the support from 2DCC-MIP. And also I have- again we have typically when
we are growing a wide range of quantum material, we have the opportunity to collaborate with
many other research groups, as shown in this maps and the list of names. And I will also acknowledge 2DCC for the opportunity. Thank you. [applause]
Dressler: We have a few questions online. The first is, why is a T-shape ideal for the
floating zone method. Mao: Ok so if it is I-shape, I-shape means
a very thin melting zone, right? So when that happens, it means the viscosity
is very low. So the liquid will cool down very quickly,
right? When this happens, you cannot- It will nucleate,
but cannot continue to grow. So this is related to viscosity now. The T-shape has a proper viscosity. So you will hold the liquid phase in the hot
zone for a while, and then gradually cool it down. If we cool down very quickly. It just drops out. It will not crystallize. So that is why T-shape is important
Dressler: The second one online is how can we get rid of forming multi-component twins,
especially in solvent evaporation method at low temperature. Mao: How we get rid of what? Dressler: How we get rid of forming multi-component
twins. Mao: Multi-component twins, in solution growth? Dressler: In solvent evaporation. Mao: It is unavoidable that this happens quite
often. As long as crystal size is large, you can
always separate again. So they could be twins, the different crystals
oriented along different directions, but they are combined, right? And then you can cut it, right? Just cut it apart. But the growth itself we cannot control that. Dressler: Questions here on site? Volunteer 1: So the slow cooling method, you
said there would be less modification when the solution is cooled slowly, right? Mao: Yes slow cooling. With fast cooling you will go to the unstable
supersaturation. Volunteer 1: So would it also be applicable
for a solid substance, like if you have an amorphous substance and you heat it to a high
temperature and cool it really slow. Would that be – ?
Mao: Yes, that also � so if you have polycrystalline material with some micro-grains. And then you have to cool down very slowly
for long time. Much longer time than solution growth. And then if you are lucky you also get good
single crystals. Yeah we did that before. Yes, slow cooling. Volunteer 1: What sort of time scale? Mao: Couple of weeks, yes. We have one growth takes like one month. Volunteer 2: So you mentioned a lot of flux
mediums. How about using ionic liquids for low temperature
solution growth? Mao: Ionic liquids like sodium chloride, this
ionic liquid? Volunteer 2: Yeah, but more like organic. Mao: Yeah oh ionic organic can also be used. Volunteer 2: So is there any difference — you
would use for low temperature, right? Mao: Yeah right, so organic solution medium
for low temperature growth. Yeah I am not sure if they can be used for
high temperature solution growth. But from literature, what I learned is the
organic solution is usually be used for growing crystals at low temperature, usually organic
crystals. Not inorganic. Volunteer 2: But organic ionic crystals can
result in inorganic molecules with metals � could you use that for low-temperature
crystal growth. Mao: Yeah I think they can be used for low-temperature
growth, but I am not sure if they can do high temperature growth. So from literature I have not seen anyone
use organic solution. I have only seen inorganic solution to grow
materials with high melting point. Yeah, but this is very interesting to exploit. It might be useful. Volunteer 3: When a synthesis takes a couple
of weeks, that is kind of a long time to wait around. I wonder, if it takes that long for a single
experiment, are you ever sure that you have optimized your synthesis or do you just get
to a point where you say, �that is good enough�? Mao: Oh yeah, that is a good question. If it is solution growth, you know, a single
batch you need to wait a couple of weeks to see what happens, and then you — and that
is what is not efficient. What we do is make multiple batches, with
6 batch or even 10 batch at the same time with different � we put them in many different
furnaces and we grow them with different temperatures and different conditions, right?. And then we compare them. And then we can find what conditions are optimized,
so that will be much more efficient. But for floating zone growth, you cannot do
multiple growths at the same time. It is only one growth at a time, right? And usually one growth takes one week. You need a couple of days to make a rod. And then growth may take, even a fast growth,
takes one day, right? For slow growth, if you use the growth speed
of 1 mm/hr or even below 1 mm/hr it takes a week. Of course you cannot stay there for a full
week, right? You still need to look at it from time to
time, right? So in that case you need to use a remote controller
� to look at growth. Dressler: Lets thank Dr. Mao.
[applause] https://www.youtube.com/watch?v=7yhmJHs7Njs&t=609s

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