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