Yazid Yaakob

Field emission properties of chemical vapor deposited individual graphene

Here, we report field emission (FE) properties of a chemical vapor deposited individual graphene investigated by in-situ transmission electron microscopy. Free-standing bilayer graphene is mounted on a cathode microprobe and FE processes are investigated varying the vacuum gap of cathode and anode. The threshold field for 10 nA current were found to be 515, 610, and 870 V/μm for vacuum gap of 400, 300, and 200 nm, respectively. It is observed that the structural stability of a high quality bilayer graphene is considerably stable during emission process. By contacting the nanoprobe with graphene and applying a bias voltage, structural deformation and buckling are observed with significant rise in temperature owing to Joule heating effect. The finding can be significant for practical application of graphene related materials in emitter based devices as well as understanding the contact resistance influence and heating effect.

Visualizing copper assisted graphene growth in nanoscale.

Control synthesis of high quality large-area graphene on transition metals (TMs) by chemical vapor deposition (CVD) is the most fascinating approach for practical device applications. Interaction of carbon atoms and TMs is quite critical to obtain graphene with precise layer number, crystal size and structure. Here, we reveal a solid phase reaction process to achieve Cu assisted graphene growth in nanoscale by in-situ transmission electron microscope (TEM). Significant structural transformation of amorphous carbon nanofiber (CNF) coated with Cu is observed with an applied potential in a two probe system. The coated Cu particle recrystallize and agglomerate toward the cathode with applied potential due to joule heating and large thermal gradient. Consequently, the amorphous carbon start crystallizing and forming sp2 hybridized carbon to form graphene sheet from the tip of Cu surface. We observed structural deformation and breaking of the graphene nanoribbon with a higher applied potential, attributing to saturated current flow and induced Joule heating. The observed graphene formation in nanoscale by the in-situ TEM process can be significant to understand carbon atoms and Cu interaction.

Transfer free graphene growth on SiO 2 substrate at 250 °C

Low-temperature growth, as well as the transfer free growth on substrates, is the major concern of graphene research for its practical applications. Here we propose a simple method to achieve the transfer free graphene growth on SiO2 covered Si (SiO2/Si) substrate at 250 °C based on a solid-liquid-solid reaction. The key to this approach is the catalyst metal, which is not popular for graphene growth by chemical vapor deposition. A catalyst metal film of 500 nm thick was deposited onto an amorphous C (50 nm thick) coated SiO2/Si substrate. The sample was then annealed at 250 °C under vacuum condition. Raman spectra measured after the removal of the catalyst by chemical etching showed intense G and 2D peaks together with a small D and intense SiO2 related peaks, confirming the transfer free growth of multilayer graphene on SiO2/Si. The domain size of the graphene confirmed by optical microscope and atomic force microscope was about 5 μm in an average. Thus, this approach will open up a new route for transfer free graphene growth at low temperatures.

Room-Temperature Fabrication of Au- and Ag-Incorporated Carbon Nanofibers by Ion Irradiation and Their Field Emission Properties

We have demonstrated the growth of Au- and Ag-incorporated carbon nanofibers (CNFs) at room temperature by Ar+ bombardment on graphite surfaces with simultaneous Au and Ag supply. The evolution of their morphology and its effects on field emission properties were investigated. The structure and density of the grown CNFs depended on the metal supply rate. The ion-irradiated surfaces with excess metal supply featured sparsely distributed conical protrusions and a wall-like structure, while the surfaces irradiated with appropriate metal supply produced densely distributed CNF-tipped cones and a needlelike structure. Compared with Ag supply, Au supply yielded fewer CNFs in terms of number density. Thus, the CNF number density was controllable by adjusting the metal supply rate and metal species. A lower threshold field and a higher emission current density were achieved in the field emission of both metal-incorporated CNFs than of pristine CNFs (without metal incorporation). Thus, it is believed that metal-incorporated CNFs are promising for practical field emission device applications.

In situ transmission electron microscopy of Ag-incorporated carbon nanofibers: The effect of Ag nanoparticle size on graphene formation

We have studied graphene formation from a single Ag-incorporated carbon nanofiber (CNF) during electron emission using in situ transmission electron microscopy. The formation of graphene from the Ag-incorporated CNF structure was observed under a high current of between 900 nA to 2.03 μA during field and thermal electron emission. Joule heating during the process generated an increased temperature, estimated at approximately 440 K to 1030 K, leading to the transformation of a significant amount of the amorphous carbon surrounding Ag particles to a graphene structure, and to the nearly simultaneous evaporation of Ag particles. This evaporation interrupted the thermal electron emission process, thus leading to a decrease of the emission current to ∼300 nA. Also, graphene stopped forming after the Ag particles had evaporated. In this paper, the effect of Ag particle size on its ability to catalyze the fabrication of high-quality graphene are discussed.

In situ fabrication of graphene from a copper–carbon nanoneedle and its electrical properties

Herein, we present a direct observation of the formation of graphene from a single copper–carbon nanoneedle (Cu–CNN) during the measurement of current–voltage (I–V) and direct heating via in situ transmission electron microscopy (TEM). Significant structural transformation of Cu–CNN was observed with an applied potential in a two probe system. Under a high current flow between 4.9 μA to 49.0 μA, the Cu nanoparticles melted and evaporated due to Joule heating. The amorphous carbon began crystallizing and transformed into sp2 hybridized hollow graphitic carbon, which was catalyzed by the dispersed Cu nanoparticles. The temperature generated during the current flow was estimated to be 1073 K, as revealed by an in situ TEM heating experiment. The graphene nanoneedle formed exhibited a high current density of 106 A cm−2, which is comparable to Cu in normal interconnect applications. Thus, the graphene nanoneedle formed will be promising for future alternative interconnect materials.

Room temperature fabrication of 1D carbon-copper composite nanostructures directly on Cu substrate and their field emission properties

This paper demonstrates a carbon-copper (C-Cu) composite nanostructure directly fabricated on a copper (Cu) substrate using the Ar+ ion irradiation method at room temperature. The morphology of C-Cu composite was controlled by a simultaneous carbon supply during ion irradiation. Conical protrusions formed on the surface of the Cu substrate with the low carbon supply rate (RC), whereas high RC area prominently produced nanoneedle structures. The field electron emission (FEE) tests demonstrated significant improvement between conical protrusions and nanoneedle structures, where the emission current increase from 5.70 μAcm−2 to 4.37 mAcm-2, while the turn-on field reduced from 5.90 to 2.00 Vμm−1.

In situ TEM visualization of Pd assisted graphene growth in nanoscale

Pd is a unique substrate to explore graphene growth. Pd is a well-known “carbon sponge” which have potential to grow graphene with semiconducting properties in nature. Here, we reveal a solid phase reaction process to achieve Pd assisted graphene growth in nanoscale by in-situ transmission electron microscope (TEM). Significant structural transformation of amorphous carbon nanofiber (CNF) incorporated with Pd is observed with an applied potential in a two probe system. The Pd particle recrystallize and agglomerate starting from the middle part of CNF toward the end part of CNF with applied potential due to joule heating and large thermal gradient. Consequently, the amorphous carbon start crystallizing and forming sp2 hybridized carbon to form graphene sheet from the tip of Pd surface. The observed graphene formation in nanoscale by the in-situ TEM process can be significant to understand carbon atoms and Pd interaction.

Visualization of graphene formation in nanoscale by in situ transmission electron microscopy: A Review

In situ transmission electron microscopy (TEM) observation of the graphene synthesis by solid phase reaction is dealt with. For this purpose, metal-included and pristine (metal free) amorphous carbon nanofibers (CNFs) were fabricated on an edge of a graphite foil by Ar ion irradiation (Ar ion irradiation method) with and without a supply of metals at room temperature, respectively. In Ar ion irradiation method, any kinds of metals could be included in CNFs. Cu-coated pristine CNFs were also prepared. The graphite foils thus prepared were cut into small pieces to be mounted directly on a TEM sample holder equipped with a piezo-controlled nanoprobe to, measure their current-voltage characteristics in direct current (DC) and field emission (FE) modes. Depending on the included metal element in the CNFs, different types of nanocarbons, such as graphene, ring-shaped graphene, and carbon nanotubes (CNTs), were formed from amorphous CNF during the electron current flow by solid phase reaction. Here, dependence of included metal on the nanocarbon formation is reviewed, together with the visualization of graphene sheet formation in nanoscale during the electron current flow for the Cu-coated pristine CNFs. It is believed that the graphene formation by solid phase reaction is essential also for the growth area (position) control of graphene nano-ribbon.