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Tech Front: New Method Creates Precise Graphene Nanoribbons

 

A team of researchers from the Department of Energy’s (DOE) Lawrence Livermore National Laboratory (Berkeley Lab) and the University of California, Berkeley, has designed a new precision method of synthesizing graphene nanoribbons from molecular building blocks. The research, which created nanoribbons with enhanced properties, could be used in future electronic circuitry.

These nanoribbons, which are narrow strips of graphene, exhibit properties that make them a prime choice for future nanoelectronic technologies, according to the researchers. The results of the research were published in a paper entitled “Molecular bandgap engineering of bottom-up synthesized graphene nanoribbon heterojunctions,” in the journal Nature Nanotechnology.

“This work represents progress towards the goal of controllably assembling molecules into whatever shapes we want,” said Mike Crommie, senior scientist at Berkeley Lab, professor at UC Berkeley, affiliated with the Kavli Energy NanoScience Institute, and a leader of the study. “For the first time we have created a molecular nanoribbon where the width changes exactly how we designed it to.”

Bottom-up synthesis of graphene nanoribbons (left of image) from molecular building blocks 1 and 2. (a) The resulting ribbon, or heterojunction, has varied widths as a result of different width molecules 1 and 2. Scanning tunneling microscope image (right) of graphene nanoribbon heterojunction, with larger-scale inset of multiple ribbons.

In the past, scientists created nanoribbons that have a constant width, but this time the research team tried a new approach. “We wanted to see if we could change the width within a single nanoribbon, controlling the structure inside the nanoribbon at the atomic scale to give it new behavior that is potentially useful,” Crommie said.

Felix Fischer, professor of chemistry at UC Berkeley, also affiliated with the Kavli Energy NanoScience Institute, who jointly led the study, designed the molecular components to find out whether this would be possible. Fischer and Crommie discovered that molecules of different widths can be made to chemically bond so that the width is modulated along the length of a single resulting nanoribbon.

Nanoribbon synthesis has mostly involved etching ribbons out of larger 2D sheets of graphene. However, this lacks precision, Fischer said, and each resulting nanoribbon has a unique, slightly random structure. Another method has been to unzip nanotubes to yield nanoribbons, which produces smoother edges than the “top-down” etching technique, but it is difficult to control because nanotubes have different widths and chiralities.

“What we’ve done that is new is to show that it is possible to create atomically-precise nanoribbons with non-uniform shape by changing the shapes of the molecular building blocks,” said Crommie. While the team has shown how to fabricate width-varying nanoribbons, it has not yet incorporated them into actual electronic circuits. Crommie and Fischer hope to use this new type of nanoribbon to eventually create new device elements, including diodes, transistors, and LEDs that are smaller and more powerful than those in current use. Ultimately they hope to incorporate nanoribbons into complex circuits that yield better performance than today’s computer chips. In this effort, the researchers are collaborating with UC Berkeley electrical engineers.

This research effort was supported by the Office of Naval Research BRC Program (molecular synthesis and characterization); the DOE Office of Science (instrumentation development, STM operation and simulations); and the National Science Foundation (image analysis, theory formalism).

To view an abstract of the paper, see http://www.nature.com/nnano/journal/v10/n2/full/nnano.2014.307.html.  


SME Tech Papers: Learn More & Do More

Holemaking a Fundamental, Challenging Process

Ages after primitive peoples pounded, bent and heated malleable metals into tools came the evolution of using a tool to pierce a flat metal piece or to remove material from a deep solid—and the process of holemaking was born.

“To qualify as a good machinist is to be able to consistently make holes in exactly the right place—exactly to size, straight, and with good finish. … After most of the usual miseries have been experienced, then there comes a time when a hole driller gets involved in what is known as ‘precision holes,’” and there are many factors that have to be heeded to produce quality holes, summarizes SME Technical Paper TP62PUB6.

The paper describes some of the general techniques of making precision holes, such as boring and reaming, and discusses factors that must be considered regardless of how simple making holes might seem. For example, bushings guide reamers to locate holes in the correct position, but due to the close fit needed, ample coolant must be provided to the assembly or the reamer will seize in the bushing. As expected, the paper reports good results with carbide bushings and carbide reamers and coolant.

Gundrilling

Another accepted way to make a precision hole is by gundrilling, which as noted in TP52PUB39 is still used most commonly by firearms manufacturers (hence its name) but also for oil and gas, engine and plastic injection mold components. By virtue of a single cutting edge (“…basically a one-sided, single-lip boring tool”) and guidepads that burnish the drilled surface, gundrills produce deep, straight, precise holes.

“For gundrilling, the start of the hole is the most important issue,” as explained in TP52PUB39. With, for example, a ½" (12.7-mm) diameter rod 20" (51-cm) long with a tool bit on the end, a starting or guiding device is needed—a bushing. “After the cutting edge or the head in the gundrill has passed in the work past the depth of the bushing, then the drill produces its own guide bushing and it is continued indefinitely.” Again, carbide helps produce even better holes.

A gundrill tends to break up chips into smaller pieces, aiding their removal but necessitating the need to get rid of them quickly. Coolant is applied under pressure through a hole in the drill, and chips are flushed away by a groove (flute) along the tool.

As stated in TP63PUB190, a companion paper to TP62PUB6, “a major problem in gundrilling is the necessity of forcing chips out of the holes with oil pressure.” This issue applies to deep as well as to short or small-diameter holes, which are described as a new application for gundrills in the 1962 paper. (To make us feel old, an interesting case history in the paper covers making parts for “the current model of the Polaroid camera shutter case.”)

The coolant delivery hole is clearly visible on several of these gundrills.

In deep-hole drilling, coolant enters through a hole running the length of the drill; in short holes, coolant is added through the bushing into a restricted-length flute just above the tool-workpiece interface. The restricted flute length in short-hole drilling makes it possible to pump greater volumes of cutting fluid to the tip at much lower pressures. Whether for deep or short holes, the fluid lubricates and cools the drill end and then washes the chips up and out through another flute.

Eight detailed case studies are given in TP58PUB23 on production drilling and reaming of precision holes with gun-type tools. Types of drills and reamers, setup for gundrilling, coolant flow, drill sharpening and tool life are covered. Gundrilling equipment for exotic materials, including a machine with synchronized rotation of the drill and feed, is described in TP66PUB166. Beryllium is among the most difficult exotic materials to handle but is an important material in atomic reactors. Its chips are small, extremely abrasive particles that float in coolant, so filtration is an issue.

Three-Part Overview

A trio of papers from 1969 presents perspectives on the manufacturing of holes from the administrative, engineering and shop-floor points of view. The authors, all from the representative departments at Zagar Inc. (www.zagar.com; Cleveland), explore techniques in the holemaking process, inspection and tolerance problems and solutions, design specifications and equipment instructions, and equipment building.

In TP69PUB198, the Zagar president noted that because the first machine tool that many people personally are involved with is probably a drill press to make holes, everyone can feel like they are an “expert.” The task, though, is if the personal work technique of some individual “does produce a better job, then it is up to administration to ferret out the why’s and wherefore’s and change the standard practice instructions to incorporate new and better developments.”

From the engineering side, in TP69PUB13, is repeated the still-ultimate goal to “develop tooling packages that aim to give the customer the best production pieces for the least expenditure of tooling dollars.” In TP69PUB199, the plant superintendent emphasizes taking advantage of viewing the progress of particular projects to “see the results of your
engineering efforts.”

Coolant Pressure

Pressure-coolant tools include drills and reamers for precision hole production. TP60PUB10 details a two-flute center-cut type of gundrill, a single-flute gundrill and a two-flute type of pressure-coolant reamer. By the same chief-engineer author as TP58PUB23, from Star Cutter Co. (www.starcutter.com; Farmington Hills, MI), the paper describes the development of multifluted tool designs for the most efficient production and gundrilling processes combined with pressure-coolant reaming processes for efficient and low-cost production of precision long, short and stepped holes in parts.

Papers TP66PUB26 and TP67PUB3 offer insights into the effects of pulsating coolant pressures in oil-hole drills. There was a substantial increase in the metal removal rate compared with steady flow, dammed-up chips were eliminated and higher feed rates helped chip removal. The 1967 paper provided a progress report on more than 300 installations of a “jet pulser” pump system on machines processing a variety of materials.

TP67PUB207 briefly outlines how advances in coolant techniques such as the application of vapor coolant and pulsating coolant contributed to the wider use of the coolant-hole drill. A prototype oil-hole (coolant-fed) drill press for experimental drilling with drills up to ½" (12.7-mm) diameter is described in TP70PUB64. Instrumentation, drill point configuration, coolant, drilling technique and hole condition are covered.

Research Honor Highlights Holemaking

In June, the 2015 NAMRI/SME S.M. Wu Research Implementation Award will recognize Dr. John S. Agapiou of General Motors R&D (Warren, MI) for patented and commercialized innovative holemaking tools to reduce the number of passes required to drill and finish complex bores in powertrain components. The four-flute and modified three-flute solid carbide drills were invented and developed with the sole purpose to produce top-quality bores in one or two passes as opposed to the traditional three to six passes, with sizable savings in cycle time, investment cost, tooling and maintenance.

The Wu Award highlights research was presented at an annual North American Manufacturing Research Conference (NAMRC) that has been implemented in industry with significant impact. Agapiou’s paper, “An Evaluation of Advanced Drill Body and Point Geometries in Drilling Cast Iron,” was presented at NAMRC in 1991.



TechFront is edited by Senior Editors Patrick Waurzyniak, pwaurzyniak@sme.org, and Ellen Kehoe, ekehoe@sme.org.

 

More than 100 papers in SME’s knowledge collection focus on aspects of holemaking, including applications of EDM/ECM, laser and ultrasonic techniques. SME Technical Papers (coded as TP…PUB…) and search options for the collection are available at http://tinyurl.com/SearchTPs.

 

This article was first published in the April 2015 edition of Manufacturing Engineering magazine. Click here for PDF.


Published Date : 4/1/2015

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