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Nanotube fountain pen: Towards 3D manufacturingof metallic nanostructures

http://dx.doi.org/10.1016/j.carbon.2015.01.0430008-6223/� 2015 Elsevier Ltd. All rights reserved.

* Corresponding author: Fax: +1 517 3531980.E-mail address: ldong@egr.msu.edu (L. Dong).

Zheng Fan a, Xinyong Tao b, Xudong Fan c, Xiaobin Zhang d, Lixin Dong a,*

a Department of Electrical and Computer Engineering, Michigan State University, East Lansing, MI 48824, USAb College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, Chinac Center for Advanced Microscopy, Michigan State University, East Lansing, MI 48824, USAd Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

A R T I C L E I N F O A B S T R A C T

Article history:

Received 9 September 2014

Accepted 23 January 2015

Available online 31 January 2015

The additive manufacturing of structures and devices in nanoscale yields the possibility of

extending the functionalities of nanoelectronics and developing the alternative nanofabri-

cation technologies for the post-silicon era. The nanorobotic spot-welding that is imple-

mented by the transporting of mass inter/intra through the carbon nanotube has been

demonstrated in previous works. However, the continuous feeding of the metal mass in

nanoscale, as well as the servo control of such mass feeding rate and 3D path of the nano-

tube injector is still unsolved. In this report, we demonstrated a nanotube fountain pen

(NFP). It is equipped with a metal-filled carbon nanotube (m@CNT) as a pen-tip injector

to ‘‘write’’ and a nanotube network as an ‘‘ink reservoir’’. In this way, it is able to provide

the possibility of fabricating 3D nanostructures in a continuous fashion. The resolution

of NFP depending on the diameter of the nanotube-injector and the mass flow rate of it

can be modulated by external electrical driven energy. The development and control of

NFP provide a new additive manufacturing technique in nanoscale, and thereby enables

potential to develop a NFP-based metallic 3D nanoprinting system for the large-scale,

high-throughput manufacturing of nanodevices.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Additive manufacturing (AM) techniques are approaches to

the fabrication of structures and devices, especially 3D ones,

by depositing materials onto a substrate, which have the

advantages of distributed manufacturing and building from

bottom up [1–5]. Based on the concept of the AM, several

novel fabrication methods as well as some interesting tools

were developed, such as the 3D printer and 3Doodler (the

world’s first 3D printing pen) [6].

Moreover, directly manufacturing with metals, treated as

the final frontier in AM techniques, has achieved remarkable

advancement in recent years [7–12]. Compared with conven-

tional metal component manufacturing process that usually

lasts from weeks to months and lacks dynamics in product

design, AM techniques allow the lean manufacturing of the

machine parts [5,13,14], indicating a new revolution in the

manufacturing industry. The recent progress in the miniatur-

ization of the electromechanical system raises the question of

whether the 3D metal printing can be realized at the nano-

scale or even atomic level, which is also the core of bottom-

up nanotechnology defined as molecular manufacturing [15].

At the nanometer scale, unlike conventional nanolitho-

graphic technologies [16–18], additive nanolithography

C A R B O N 8 6 ( 2 0 1 5 ) 2 8 0 – 2 8 7 281

typically involves a high energy beam or a scanning probe.

Among the beam-based methods, the direct laser writing

(DLW) [19] has the advantage of fast prototyping speed, but

this technique cannot be used to deposit metal materials.

Electron-beam-induced deposition (EBID) [20] and the

focused-ion-beam chemical vapor deposition (FIB-CVD) [21]

have higher resolution and allow multi-material such as met-

als, and high resolution deposition, but their limitations of

energy and large consumption of time constraints their appli-

cations to the 3D nanodevice batch fabrication. Other solu-

tions that have been proposed are probe-based additive

nanolithographic methods, such as dip-pen nanolithography

(DPN) [22] and polymer pen lithography (PPL) [23]. They are

developed with advantages of high-throughput and lower

energy consumption than the beam-based methods. How-

ever, no metallic deposits have been obtained.

Therefore, the 3D metal nanostructure AM is still a tech-

nical challenge for the existing additive nanolithographic

methods. Here we propose a nanotube fountain pen (NFP)

worked as a 3Doodler in nanoscale for direct writing with

metallic ‘‘inks’’. The NFP is consisted of a reservoir and a

nanotube injector. As shown in Fig. 1a, a single copper-filled

carbon nanotube (Cu@CNT) is used as the pen tip and the

‘‘ink’’, i.e., the mass reservoir is attached with the pen injec-

tor. The pen tip transports attogram metal from the mass

reservoir onto a conductive substrate to form the 3D

nanostructures. As a pen-type lithography method not only

benefits from the availability of metallic structures, the NFP

also has the advantage of continuous writing ability as

compared with a nanotube pencil [18] and the DPN. This

resembles the development of their macroscale counter-

parts, from pencils, to dip pens, to fountain pens, and to

the 3Doodler. Three main issues of the development of

NFP were addressed in this report: the continuous mass

feeding, the control of the mass flow rate, and the position

servo control.

Fig. 1 – NFP (a) the setup of a NFP. (b) I–t characterization during t

as ‘‘OFF’’ (external bias: 1.0 V) and the state after the mass mig

initial mass inside the nanotube injector is 1.9 fg, while the dep

mass). (c) Inner mass leaked out of carbon shell and accumulate

can be viewed online.)

2. Experiment

The Cu@CNTs that used as NFP prototype were synthesized

by a one-step CVD method [24]. We have experimentally

investigated the feasibility of NFP in fabricating complex

metal nanostructures from 0D to 3D with a manual servo

position control. The manipulation of the NFP is realized by

using a scanning tunneling microscope (STM) built inside a

transmission electron microscope (TEM) holder (FM2000E

STM-TEM holder, Nanofactory Instruments AB). As the NFP

attached on a conductive substrate, a circuit can be estab-

lished. By controlling the external input voltage, the writing

or reloading of the NFP could be realized.

As shown in Fig. 1b, under a stable bias (1.5–2.5 V), the

inner ink/mass continuously flows out of the NFP. It has been

estimated based on the geometry and the density of copper

that the initial mass inside the nanotube injector is 1.9 fg,

while the deposited mass is 29.0 fg (about 15 times more than

the original mass). The extra mass is from the ink/mass res-

ervoir: the nanotube network that attached to the CNT

injector.

3. Discussion

3.1. NFP writing process

The current–time (I–t) behavior during the continuous mass

delivery is demonstrated in Fig. 1b. The mass delivery started

when the external bias reached 1.5 V that increased from 0 V

at increments of 0.1 V. In order to prevent the electric break-

down of the nanotube that was caused by the sharp increas-

ing of the current [25–26], the current value was restricted

around 90 lA with the external bias of 1.5 V. The nanotubes

served as the bridge through which the mass could be trans-

ferred from the CNT network (the mass reservoir) to the injec-

tor, and finally be deposited onto the object [27,28].

he NFP ‘‘writing’’ process. The initial state (0–300) is defined

ration starts as ‘‘ON’’ (external bias: 1.5 V). Insets show the

osited mass is 29.0 fg (about 15 times more than the original

d on the side-wall of the CNT. (A color version of this figure

Fig. 2 – The schematic of NFP ‘‘writing’’. (a) The initial I–V curve of NFP in the ‘‘OFF’’ state shows a non-ohmic/charging

characteristic. The CNT–Substrate (C–S) junction and the CNT–CNT (C–C) junction are placed at the two ends of the nanotube

respectively. (b) The I–V performances in the C–C and C–S junction. (c) The generation of super-fine nanowire by using a

Cu@CNT. Inset shows a hole on the tip of CNT. (A color version of this figure can be viewed online.)

282 C A R B O N 8 6 ( 2 0 1 5 ) 2 8 0 – 2 8 7

As shown in Fig. 1c, because the rate of the mass trans-

porting from reservoir into the front NFP injector was higher

than that of the mass flowing out, the pressure in the CNT

was accelerated due to the increasing mass volume in the

tube. It is highly possible that this is due to the nanotube

shell’s occasional defects or flaws that were generated during

the synthesis process, the inner mass would leak out of the

tube from these flaws with the increasing of the inner pres-

sure. The leaked mass gathered on the side wall of CNT and

formed a bubble. The bubble did not migrate on the exterior

of the tube. Instead, it would stay in the original leaking spot.

To analyze the NFP writing process, we defined the initial

state (0–300 s) as ‘‘OFF’’ (external bias: 1.0 V) and the state

after the writing begins as ‘‘ON’’ (external bias: 1.5 V). The cur-

rent–voltage (I–V) curves at ON and OFF contain important

information for deciphering the nature of mass accumulation

and migration. A non-ohmic/rectifying contact exists at the

CNT injector–Substrate (C–S) interface or the CNT injector–

CNT network (C–C) interface (Fig. 2a). To identify the specific

roles of each interface, as well as the contributions of them

to the mass accumulation, we have carried out an experiment

to verify the performance of each interface. We used different

CNTs to make contact with CNT and Au surfaces, and then

obtained the I–V curves of each of them. The linear I–V curve

demonstrated that both the interfaces (CNT network–CNT

injector, CNT injector–CNT) were ohmic (Fig. 2b). However,

C A R B O N 8 6 ( 2 0 1 5 ) 2 8 0 – 2 8 7 283

the contact between the CNT injector and the Au object

revealed a non-ohmic characteristic, which suggested that

there was at least one interface that was limited by the

non-ohmic transport. Due to the contact resistance and other

effects (e.g., metal oxidation or bad contact condition) that

may exist on the CNT surface, the non-ohmic I–V behavior

was generated at lower external energy (OFF) state. When

the bias voltage was increased and the system was turned

ON, the non-ohmic junction (C–S) would collapse and shift

to an ohmic contact. The previously mentioned metal accu-

mulation process (Fig. 1c) further confirmed the junction

model: as the current was sharply increased at the transition

state between OFF and ON, the electromigration-driven force

was increased accordingly [27,29]. Due to the equal energy

barrier at the C–C junction and the electromigration effect

from cathode to anode [30,31], the mass from the reservoir

continuously migrated into the injector (Fig. 1b). Comparing

the I–V curves at ON and OFF states, we concluded that at

the OFF state, the C–C junction was ohmic and C–S junction

was rectified. At the ON state, the blocked junction was col-

lapsed because of the higher bias voltage and enabled the

mass transport. However, what is the sequence of an initially

opened C–S junction? We selected a Cu@CNT with a hole as

starting material for the ‘‘writing’’ test (Fig. 2c). This time,

no transition between the OFF and ON state occurred because

of the direct contact between the inner metal and the probe

surface. In other words, the writing began instantly once

the connection was established. This produced a superfine

metal nanowire. In addition, the generated nanowire had

the diameter of 11 nm, which was comparable to the hole

diameter of 19 nm (Fig. 2c, inset). The writing tests demon-

strated the fabricating a superfine metal nanowire with the

diameter controlled by a hole on the nanotube, which showed

Fig. 3 – The mechanism analysis of NFP ‘‘writing’’. (a) Mass mig

illustrates the competition between electromigration force and

mass flow rate study on a Cu@CNT combined with the reservoir.

the mass flow rate and the acceleration current. (A color versio

the potentials of reducing the diameter of the nanowire down

to a few nanometers.

3.2. NFP writing mechanism

The remaining question is what factor drives the mass to

flow. Prior works in the area have showed that the electromi-

gration effect and the thermal gradient effect are the main

driving forces of the mass flowing [29,32–36]. These two

effects were also observed in the NFP writing process.

Fig. 3a shows the mass delivery inside a NFP without the res-

ervoir. When the external bias reaches 2.0 V (0 s), the dark

contrast and the boiling sign indicate the molten state of

the copper wire. As the voltage is increased to 2.2 V (7 s), the

mass in the center suddenly evaporates, clearly revealing

the thermal effect [37]. Nevertheless, by further increasing

the voltage in 0.1 V increments until the external bias reaches

a higher value (2.5 V, 11 s), the mass begins to migrate from

the upper part to the lower portion of the tube, following

the direction of the electromigration force. As we carried

out the experiment with the same setup but using a nanotube

attached with a reservoir, the mass migrating procedure was

similar to the previous experiment. However, the difference

between these two experiments is that when the reservoir

was attached to the upper side (cathode), the mass from the

reservoir continuously migrated into the nanotube following

the direction of the electron migration and finally filled up

the gap in the nanotube. Fig. 3b illustrates that the relation

between the mass migration inside the tube and the time is

given as: m = 0.0168t2 � 0.0681t + 1.9971 (ag), the external bias

here was remain in 2.5 V. Thus, the mass flow rate can be

expressed as: dm/dt = 0.0336t � 0.0681 (ag/s), indicating a lin-

ear mass flow.

ration from cathode to anode illustrates. The process

thermal gradient force during the mass transport. (b) The

(c) The model of the mass transport. (d) The relation between

n of this figure can be viewed online.)

Fig. 4 – NFP ‘‘writing’’ (a) the mass flowing schematic at the very beginning of NFP writing. (b) Correlation of the NFP diameter

D with the external electric energy. (A color version of this figure can be viewed online.)

Table 1 – Comparison of power threshold of NFP withdifferent diameters.

D (nm) I (lA) V (V) P (lW)

26.9 44.3 1.2 53.237.5 91.1 1.5 136.747.2 193.8 2.0 387.656.0 205.5 2.3 472.769.9 331.5 2.5 828.8

284 C A R B O N 8 6 ( 2 0 1 5 ) 2 8 0 – 2 8 7

Based on the experimental observations, we propose a

general model (Fig. 3c) to elucidate the dynamics during the

mass flow. At the beginning of the mass evaporation process,

the NFP is at the OFF state. Thus the low current value pro-

duces a lower electromigration force. If we assume the mass

migration is driven by this effect, the mass in Fig. 3a and b

should consistently move from cathode to anode; however,

the mass evaporating in the center does not move in one sin-

gle direction. Because the center of the nanotube is always

the hottest spot [26,37,38], part of the mass is evaporated

and diverted from the center (the hotter part) to the periphery

(colder part) by thermal gradient force. Therefore, the thermal

gradient effect dominates the mass migration at the begin-

ning of the NFP writing. The increase of the external bias

turns the NFP from OFF to ON state, and the current sharply

increases accordingly (Fig. 1b). During this process, the elec-

tromigration force is strengthened and overcomes the ther-

mal force. Then the electromigration effect ultimately

dominates the mass migration and drives the mass to flow

from cathode to anode. Moreover, the mass’ flowing out rate

of the NFP illustrates a linear correspondence to the time at

stable applied current (Fig. 3d). When the driven bias

increases at increments of 0.2 V, the current curve shows a

remarkable regular staircase with step sizes of about 1.2 lA,

and the mass’ flowing rate is accelerated accordingly. This

behavior further indicates the dominant role of the electromi-

gration effect in driving the intra/inter nanotube mass

migration.

In addition, Fig. 3d demonstrates that the diameter of a

deposited nanowire is comparable to the diameter of the tube.

Based on the molecular dynamic simulation (MDS) of the

metal atom transporting through a single carbon hexagon

ring [27], the atomic mass deposition at the very beginning

of the NFP ‘‘writing’’ could be illustrated: Firstly, there are only

a few metal atoms passing through the carbon hexagon ring

and forming the conductive filament channel between the

inner copper atoms and the probe surface; secondly, once

one or more metal atoms penetrate the carbon hole and con-

nect between the CNT and the deposition objects (Fig. 4a), the

conductivity is largely increased; finally, due to the increasing

of the electromigration force, more atoms could flow out of

the rest of the carbon hexagon rings on the tip of the nano-

tube and generate the nanowire with similar size to that of

the tube. For the nanotube with different sizes, different pow-

ers are needed to start the mass flow. Table 1 shows the elec-

trical power P (P = IV) for the tubes of different sizes to make

the mass flow out. The power–diameter (PD) curve which can

be fitted by: P = 0.2228D2 � 3.5851D � 4.6565 (lW) (Fig. 4b).

3.3. NFP writing capillary

As the nanowires were written on the object, the shape was

formed immediately thanks to the excellent thermal conduc-

tivity of the probe that cooled down the deposit instantly [39].

Reheating of the cooled-down deposit was not achieved

because the volume of the probe (tip radius: 120 nm, root

radius: 10 lm) was much greater than that of the copper

deposit. The probe served as a heat sink with essentially

capacity compared with the copper deposit. A detailed exam-

ination of the crystalline structure of the generated nanowire

can further explain their shaping process. Fig. 5a illustrates

the crystalline analysis of the generated nanowire. In the con-

nection area between the probe and nanotube (area I), the

crystalline phase of the deposited copper shows a poly-crys-

talline phase. This is attributed to the instant cool-down at

the heat sink [39], which prohibits the molten deposit from

being fully crystallized before it forms the solid state. In con-

trast, area II reveals single-crystalline phases because it is far

from the heat sink. Moreover, according to the dark contrast

and the amorphous phase seen in area III, we can confirm

the molten state at this position. Therefore, the molten state

in area III provides opportunity to shape the final structure by

manually positioning the injector.

Next, the potential practical applications of NFP will

be introduced. We will first describe representative

Fig. 5 – The applications of NFP. (a) The crystalline phase analysis of as-generated nanowire. Due to the molten state at the

connection between the deposit and the injector, it provides possible to shape the deposited structures. (b) The writing

process of the letter ‘‘N’’. By manually positioning the molten copper onto the probe can freeze it immediately, and thereby

construct the nanostructure as depicted a stick figure. (c) The writing process of the letter ‘‘L’’. (d) The demonstration of using

different sized CNTs to fabricate the nanostructures. (e) The demonstration of the modulating of deposits by adjusting the

position of injector. (A color version of this figure can be viewed online.)

C A R B O N 8 6 ( 2 0 1 5 ) 2 8 0 – 2 8 7 285

experiments with multi-NFP, and then compare the drawing

with a single NFP. Although the servo control was not

applied in the experiment, manual control of the nanowire

deposit during the NFP writing is possible. By attaching the

tip of the injector to the probe during the mass flow

(Fig. 5b), the shape of the molten nanowire deposit can be

adjusted, and the 3D nanostructure similar to the letter

‘‘N’’ can be produced eventually. In Fig. 5c, for the capital

letter ‘‘L’’, the injector was moved during the mass flow to

form the letter. Moreover, it is possible to construct a nano-

scale composition with the components of different diame-

ters (Fig. 5d). Since it is difficult to use nanotubes with

different diameters to fabricate, we have also investigated

using a single nanotube to fabricate components of differ-

ent sizes. The representative demonstration shows that

the injector moved back and forth in a small range

(Fig. 5e), trying to deposit the mass at the height as similar

as possible so that the deposits would melt together and

form a component that is much larger than the original

size of the copper core inside the tube.

3.4. NFP ‘‘ink’’ reloading

Although we can accumulate the mass from the reservoir that

contains the mass several ten times more than the original

mass in the CNT, it is still not enough to construct complex

devices, such as large scale integrated circuits. As a result,

‘‘mass reloading’’ was investigated to reload the mass into

the NFP after the mass in both the nanotube injector and res-

ervoir were used up. We selected a Cu@CNT half filled with

mass to carry out the mass reloading experiment. The

‘‘reloading’’ process (frames 1–2) occurred as the external bias

reached 1 V (Fig. 6a); however, due to the lower input driving

energy, only a small amount of mass was melted and trans-

ported, and the mass accumulation rate during 63 s was only

0.52 nm/s (Fig. 6b). However, as the bias voltage increased to

1.5 V (Fig. 6c), a significant mass reloading occurred. During

this process (frames 3,4,6 s), the mass transport rate reached

96.2 nm/s. We simulated the NFP under these two external

voltages to understand the nature of mass reloading. As the

NFP was simulated as a single Cu@CNT with the diameter of

Fig. 6 – NFP ‘‘ink’’ reloading (a) the reloading in a NFP with a lower driven energy. (b) I–t characterization of NFP ‘‘ink’’

reloading with different driven energies. (c) The reloading process with higher electrical energy, the reload process lasts for

6 s. (d) The simulation of the thermal distribution along a CNT as the external bias increased. (A color version of this figure

can be viewed online.)

286 C A R B O N 8 6 ( 2 0 1 5 ) 2 8 0 – 2 8 7

12 nm (Fig. 6d), the current-induced thermal distribution

illustrated that the high temperature portion along the tube

(red portion) was enlarged due to the increasing bias from

1.0 to 1.5 V. This allowed more mass to melt, and with the

increase of the electromigration force, the transport of this

mass was rapidly realized. Therefore, we concluded that the

increased input of electrical energy induced the melting and

migration of a large amount of mass, which provided the

potential for reloading the mass.

4. Future plan

NFP has several advantages over previous additive nanoman-

ufacturing methods. Compared with the existing beam-based

additive nanolithography process, the direct laser writing

(DLW) [19] has the advantage of high prototyping speed, but

this technique cannot be used to deposit metal materials.

While the EBID [20] and the FIB-CVD [21] have higher resolu-

tion and allow multi-material deposition including metals,

their features of energy and time consuming limit their appli-

cations in the 3D nanodevice batch fabrication. The third type

of solutions, the probe-based additive nanolithographic

methods, such as DPN [22] and polymer pen lithography

(PPL) [23], are developed with the advantages of high-through-

put and lower energy consumption than the beam-based

methods. Yet, no metallic deposits have been obtained.

For the future prospects of the NFP, the physical ultimate

can be atomic ones if a CNT encapsulated with a single metal-

lic atom chain [40] is used as pen-tip. As a preliminary estima-

tion, with a 10 nm (in. dia.) · 1 lm (length) CNT tip with a

5 lm (side) · 5 lm (height) pyramid reservoir (�0.5 million

times larger than the CNT in volume) in the cantilever, we

can write a 50-m-long line or 1000 lines with full length

(5 mm) on a 5 mm · 5 mm chip if the system achieves a

tmin = 1 nm thickness. If a cross bar (two lines) forms a device,

these are 0.25 million devices. By using a 10 · 10 array of drip-

pers, we can fabricate 25 million devices with a single filling.

If the bulk reservoir for refilling the pyramid reservoir in

the cantilevers has a 100 · larger dimension (500 lm

(side) · 500 lm (height) pyramid), we will be able to obtain

25 trillion devices—remarkably overpass the upper limit of

the state-of-art ICs without considering that the bulk reser-

voirs can also be refilled. Although the NFP develop in this

report only performs as a nanoscale 3Doodler, using an array

of nanotube pen-tips, parallel writing could be implemented.

Then, each nanotube serves as a pin of the NFP-based 3D

nanoprinter, which enables the batch fabrication of the

nanostructures.

5. Conclusions

In summary, we have demonstrated the implementation of

the continuous attogram metallic mass deposition using an

NFP and its mass flow rate/position servo control. Such imple-

mentation and control of NFP provide a new additive

manufacturing technique for nanostructure fabrication with

advantages over the existing techniques, for example, the

NFP is more efficient than EBID and FIB-CVD, and superior

to DPN, NFP can produce the metallic nanostructures. Given

that NFPs are able to be arranged in parallel in an array for

the batch writing and the deposited material of NFP is not

limited to metal [39], the NFP can be developed to a multi-

material 3D nanoprinting system, and thereby open a new

ground towards fabricating of nanodevices.

Acknowledgements

Z. Fan would like to thank Mr. John Doroshewitz for the final

polish of this paper.

This work is supported by the NSF (IIS-1054585), the

NSFC (51002138), the Natural Science Foundation of Zhejiang

Province (Grants LR13E020002 and LY13E020010) and New

Century Excellent Talents in University (NCET 111079).

C A R B O N 8 6 ( 2 0 1 5 ) 2 8 0 – 2 8 7 287

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at http://dx.doi.org/10.1016/j.carbon.

2015.01.043.

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