Quartz tuning-fork based carbon nanotube transfer into quantum device geometries
S. Blien,
1
P. Steger,
1
A. Albang,
1
N. Paradiso,
1
and A. K. H¨uttel
1, ∗
1
Institute for Experimental and Applied Physics, University of Regensburg,
Universit¨atsstr. 31, 93053 Regensburg, Germany
(Dated: 16 May 2018)
With the objective of integrating single clean, as-grown carbon nanotubes into complex circuits,
we have developed a technique to grow nanotubes directly on commercially available quartz tuning
forks using a high temperature CVD process. Multiple straight and aligned nanotubes bridge the
> 100 µm gap between the two tips. The nanotubes are then lowered onto contact electrodes,
electronically characterized in situ, and subsequently cut loose from the tuning fork using a high
current. First quantum transport measurements of the resulting devices at cryogenic temperatures
display Coulomb blockade characteristics.
I. INTRODUCTION
A fabrication technique that has led to many remark-
able observations in quantum transport is the in-situ
growth of carbon nanotubes onto pre-existing electrodes
and trenches in between them [1]. Published results
range from Coulomb blockade transport spectroscopy of
unperturbed electronic systems [2–5] all the way to high
quality factor mechanical resonators and strong interac-
tion between single electron tunneling and vibrational
motion [6–10]. A natural limitation of this technique
is that the electrode chip is exposed to the conditions
of chemical vapour deposition (CVD) nanotube growth,
typically 10 − 30 min in a gas mixture of hydrogen and
methane at 800−1000
◦
C [11]. Only few thin film materi-
als survive this process, notably platinum-tungsten com-
binations [1, 6] and rhenium or rhenium-molybdenum al-
loys [12–15]. Still, fabrication remains challenging and
the integration of more sensitive circuit elements such
as, e.g., Josephson junctions, quasi impossible.
The separation of growth and measurement chip pro-
vides a compelling alternative to in-situ growth of CNTs
[16–20]. For the subsequent transfer of the nanotubes
from one to the other, several approaches exist. While
pressing growth surfaces directly onto the measurement
chip to transfer CNTs potentially provides many viable
devices per fabrication step and allows the lithographic
selection of suitable CNTs on the target surface for con-
tacting [21, 22], the integration of clean, suspended CNTs
into complex, large-scale circuits requires a controlled de-
position of single macromolecules [18–20].
Here, we present a technique to grow clean CNTs be-
tween the two prongs of commercially available quartz
tuning forks and subsequently deposit them onto con-
tact electrodes of arbitrary material. We demonstrate
the details of the substrates, the transfer, and the cut-
ting process and show first low temperature transport
data.
∗
andreas.huettel@ur.de
(a)
4.8 mm
(b)
Co
H
2
, CH
4
960
◦
C
CNT
(c)
100 µm
(d)
1 µm
nanotube
FIG. 1. (a) Commercial quartz tuning forks before and after
removal of the metallization. (b) A thin Co layer is sputtered
onto the tips of the fork as catalyst for the carbon nanotube
growth by chemical vapour deposition. (c) Scanning electron
micrograph of a fork after carbon nanotube growth: the nano-
tubes clearly display a preferred growth direction. For better
visibility, here the entire fork surface has been covered with
Co growth catalyst. (d) Scanning electron micrograph of a
carbon nanotube crossing the gap between the two fork tips.
II. CNT GROWTH ON QUARTZ TUNING
FORKS
We start with a wafer piece containing several
commercial-grade quartz tuning forks, see Fig. 1(a). Af-
ter breaking out one or more forks, the metallic contacts
are removed using aqua regia, hot hydrochloric acid and
hot NaOH baths and successive cleaning steps of sonica-
tion and plasma ashing. Then, a nominally 1 nm thick
layer of cobalt is sputter-deposited onto the tips of a fork,
see Fig. 1(b). For such a nominal thickness Co does not
form a homogeneous film, but a randomly distributed en-
semble of Co clusters which serve as catalyst centers for
the carbon nanotube growth [23, 24].
As next step, the forks are placed on a glass plate and
inserted into the quartz tube of a CVD furnace. The
furnace is heated up under a steady flow of an argon /
2
100 μm
(b)(a)
fork
CNT
electrodes
1
2
3
4
FIG. 2. (a) Schematic of the carbon nanotube transfer: the
fork carrying a nanotube is sunk into two trenches that are
locally etched into a target chip on both sides of four gold
electrodes. (b) Optical micrograph of the target chip: four
contact electrodes and a ground plane (yellow), the elevated
center ridge carrying the electrodes (dark green), and sur-
rounding deep-etched areas (orange) are visible.
hydrogen mixture and then kept at 960
◦
C for 30 minutes
under a constant gas flow of methane and hydrogen. The
flow rates, 10 sccm CH
4
and 20 sccm H
2
, are typical for
clean CNT growth [11]. The fork is placed perpendicular
to the gas stream. As a result, the growth is directional
in the sense that CNTs grow mainly in the prong-to-
prong direction, see Fig. 1(b) and also Fig. 1(c,d), where
the entire fork surface has been covered with catalyst for
better visibility of the resulting nanotube growth.
Imaging the forks in a scanning electron microscope
after growth, we find that even with catalyst coating only
the fork tips typically up to five nanotubes or nanotube
bundles per fork are suspended over the gap between the
tips [5, 25]. To avoid damage and carbon contamination,
we do not image forks that are actually used for transfer.
In a future setup one could imagine using optical means,
as, e.g., Raman or photoluminescence imaging [26] to
count the suspended nanotubes between the fork prongs.
III. TARGET CHIP
For first tests of the transfer process, devices with four
long electrodes were prepared via optical lithography,
see Fig. 2(a) for a schematic side view and Fig. 2(b)
for a microscope top view. The substrate is highly p-
doped silicon, with a 500 nm thermally grown surface ox-
ide. On its surface, four finger-like gold electrodes are
deposited using thermal evaporation, and lift-off. The
typical width of the electrodes and the distance between
them are both 10 µm for this simplified test device. Next
to the electrodes, two rectangular areas are locally etched
to a depth of 12 µm by an anisotropic reactive ion etching
process using SF
6
and Ar. The etch depth should be as
large as possible and is mainly limited by the lithographic
resist protecting the remaining structure.
a)
micromanipulator
optics
glass plate
with fork
PCB with
electrode chip
(a)
glass
plate
wedge
quartz fork
(b)
Mini-SMP sockets
Micro D socket
(c)
(d)
(f)
(e)
1 µm
FIG. 3. (a) Transfer setup: the quartz fork is mounted on
a micromanipulator stage. It can be lowered to the target
chip, which is glued onto a printed circuit board (PCB) and
is electrically connected. The process is monitored via an op-
tical microscope with a zoom lens and a camera. (b) Detail
picture of how the quartz fork is mounted on the glass plate.
(c) Side view of the sample holder: to establish connection
to electronic devices, a second PCB with a Micro D socket is
attached. For further experiments, two high frequency ports
with Mini-SMP connectors are additionally soldered on top of
the board. (d-f) Scanning electron micrographs of a success-
fully transferred CNT: the nanotube has been cut between
each pair of outer electrodes (d, f) and now only connects the
two inner electrodes (e).
IV. TRANSFER AND CUTTING PROCESS
For the transfer, the quartz fork carrying as-grown
CNTs is attached to a glass object plate and mounted on
a micromanipulator stage, see Figs. 3(a) and (b). The
setup is adapted from the equipment combination used
in [27] to dry-stamp 2D materials. As there, a camera
combined with a zoom lens allows us to observe the tar-
get chip from the top. The base plate is modified insofar
as it clamps a printed circuit board sample holder with
a 25-pin MDM socket at the bottom, see Fig. 3(c). The
target chip is glued onto the circuit board and bonded;
the electrodes are electrically contacted during the trans-
fer process.
Using the micromanipulator stage, the quartz fork is
lowered onto the chip such that its tips sink into the
deep-etched areas on both sides of the dc contacts, cf.
Fig. 2(a). The process is monitored both optically and
electrically. On the one hand, we use the microscope
3
FIG. 4. (a) The current between the voltage biased contacts
1 and 4, see Fig. 2(a), is measured continuously while a quartz
fork is lowered onto the target chip. As soon as a CNT touches
the electrodes a finite current can flow. (b) Example back
gate voltage sweep at a bias voltage of 3 mV, recorded during
a transfer process before cutting the nanotube. This allows
estimating the type (metallic, semiconducting or bundle) of
nanotube before finally leaving it on the device. (c) Current
measured during two different voltage ramps for “cutting” a
CNT. From the shape of the resulting curves one can draw
conclusions on the transfer result, see the text.
camera to monitor the fork position during the align-
ment. On the other hand, by applying 100 mV between
contacts 1 and 4, see Fig. 2(a), we can detect a CNT
bridging the metal electrodes by simply measuring a fi-
nite current. This is illustrated in Fig. 4(a), where at
a time index of t ≈ 110 s contact is made. Back gate
voltage sweeps, see Fig. 4(b), then allow us to esti-
mate whether a semiconducting or metallic nanotube or
a nanotube bundle is contacted.
By ramping up a voltage bias and thereby the current
between contacts 1 and 2, as well as subsequently be-
tween contacts 3 and 4, while the device is in air, the
segments of the tube between these contacts can be elec-
trically cut. Example current-voltage characteristics dur-
ing this process are plotted in Fig. 4(c). The critical cur-
rent for cutting a nanotube typically lies in the range of
10 − 30 µA, consistent with the findings of Refs. [19, 20].
If at a certain point the current drops to zero in one
single step as, e.g., in the left part of Fig. 4(c), this in-
dicates that one single-wall carbon nanotube has been
cut. If the current decreases to zero in several steps as in
the right part of Fig. 4(c) the segment was a multi-wall
nanotube or bundle and the steps correspond to break-
ing the shells or nanotubes one at a time. We were able
to verify this interpretation of the number of steps in
the I-V-curves by extracting the diameter of successfully
transferred nanotubes from atomic force microscopy im-
ages at large contact distances, where the nanotubes can
touch the substrate.
If the approach of fork and target chip is not done care-
fully enough, a nanotube can be ripped off the fork tips
and then fall down to the substrate in the deep-etched
areas. Then, electrodes 1 and 2 are still electrically con-
nected via the substrate even after the nanotube segment
between them has been cut, resulting in a tail of finite
current in the I-V-curve, cf. Fig.4(c), left panel.
V. CLEANING OF THE QUARTZ FORKS FOR
RE-USE
After successful completion of the cutting process the
detached nanotube lies only over the inner contact pair
(2 and 3), as can be seen in the SEM image of Fig. 3(e).
The quartz fork can then be safely lifted and removed.
Given the chemical and mechanical stability of the tun-
ing forks, a rigorous cleaning procedure can subsequently
be applied to remove both carbon residues and cobalt
catalyst. We use plasma ashing to remove organic com-
pounds grown in the preceding CVD process, and a bath
of hot nitric acid to dissolve residues of old catalyst. Af-
ter sonication and another short plasma ashing step the
forks can be reintroduced into the fabrication cycle by
sputtering a new layer of Co catalyst.
VI. LOW TEMPERATURE
CHARACTERIZATION
After successfully transferring a carbon nanotube to a
substrate similar to the one shown in Fig. 2, we have
cooled down the device to liquid helium temperature.
The device was fabricated on a highly doped Si wafer,
such that the substrate can be electrically connected and
used as a global backgate. Fig. 5(a) shows the current
through the CNT in dependence on the gate voltage V
g
,
when 2 mV bias is applied. Several distinct gate voltage
regions can be distinguished in the figure. For V
g
< 1.8 V
the nanotube is strongly coupled to the electrodes, result-
ing in an open system. In the region 1.8 V < V
g
< 2.8 V
Coulomb blockade and single electron tunneling peaks
are visible; see Fig. 5(b) for a detail zoom. For 2.8 V < V
g
no current is flowing, indicating an electronic band gap.
A stability diagram at millikelvin temperatures of a
similar device, where a carbon nanotube was deposited as
described here, is shown in Fig. 5(c). The figure displays
the differential conductance as function of the source-
drain voltage V
sd
and a gate voltage V
g
. One can clearly
identify the characteristic diamond pattern of Coulomb
blockade regions as typically shown by quantum dots.
The stability diagram of Fig. 5(c) indicates a pre-
dominant electrostatic charging energy of approximately
E
c
= 0.3 meV, corresponding to a total quantum dot ca-
pacitance of C
Σ
= e
2
/E
c
= 530 aF. This is significantly
larger than typical values for a device with single-wall
nanotube length l = 1.4 µm and a distance to the gate
4
a) b)
0
0.2
-0.2
0 0.1 0.2
V
V
g
g
(V)
(V)
V
sd
(mV)
(c)
1.8 2.2 2.6
0
0.5
-2 0 2 4
0
4
8
V
I (nA)
g
(V)
I (nA)
dI/dV
2
(e /h)
-1
10
-2
10
-3
10
FIG. 5. (a) Characterization of a transferred CNT at
T = 4.2 K. Plotted is the current through the nanotube as
a function of the gate voltage V
g
, at an applied source-drain
voltage of 2 mV. Different parameter regions can be distin-
guished, see the text. (b) Zoom into the shaded area of (a),
displaying Coulomb oscillations of the current. (c) Stability
diagram of a transferred CNT at T = 15 mK; differential con-
ductance as function of gate voltage and source-drain voltage.
A pattern of Coulomb blockade areas with two distinct sizes
is visible.
of d = 500 nm, the values expected from the contact ge-
ometry here. The small charging energy may indicate
that multiwall nanotubes, bundles or nanotube networks
have been transferred and measured. The appearance of
an additional set of smaller Coulomb blockade areas in
Fig. 5(c) supports this, indicating a second confined elec-
tronic system. No transversal mechanical resonance was
found in transport measurements in a frequency range
of 100 kHz ≤ f
drive
≤ 500 MHz [6]. Further optimiza-
tion of the CVD parameters and the transfer procedure
to produce solitary single-wall carbon nanotubes is thus
required.
VII. CONCLUSIONS AND OUTLOOK
We have implemented a technique for carbon nanotube
transfer separating growth and measurement onto differ-
ent substrates. Nanotubes are grown on the tips of com-
mercially available quartz tuning forks and subsequently
transferred to a target chip of desired design.
As with other nanotube transfer procedures, the choice
of contact materials and circuit elements for the target
chip is much less constrained than for in situ overgrowth,
carbon nanotubes not suitable for measurements can eas-
ily be removed, and complex-structured devices can be
re-used in more than one transfer attempt. Transfer tar-
gets may range from, e.g., superconducting coplanar cir-
cuit geometries [28–30], qubit circuits [31], superconduct-
ing single electron transistors [32, 33], or ferromagnetic
contact electrodes [34], all the way to diamond crystal-
lites containing NV-centers [35].
The quartz tuning forks are standardized, macroscopic
parts that can be obtained in large numbers. In addition,
they are highly robust, and survive multiple cycles of cat-
alyst deposition, growth, nanotube transfer, and clean-
ing. This allows an easy, systematic approach towards
integrating carbon nanotubes into devices of arbitrary
complexity.
ACKNOWLEDGMENTS
The authors thank E. Weig for the initial sugges-
tion of using quartz tuning forks, and Coftech GmbH for
the quartz tuning fork wafer. Transport data has been
recorded using the Lab::Measurement software pack-
age [36]. We acknowledge funding by the Deutsche
Forschungsgemeinschaft via grants Hu 1808/1, SFB 689,
and GRK 1570.
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