Editing Nanotechnology (section)

==Fundamental concepts==
Nanotechnology is the science and engineering of functional systems at the molecular scale. In its original sense, nanotechnology refers to the projected ability to construct items from the bottom up making complete, high-performance products.

One [[nanometer]] (nm) is one billionth, or 10<sup>−9</sup>, of a meter. By comparison, typical carbon–carbon [[bond length]]s, or the spacing between these [[atom]]s in a [[molecule]], are in the range {{nowrap|0.12–0.15 nm}}, and [[DNA]]'s diameter is around 2&nbsp;nm. On the other hand, the smallest [[cell (biology)|cellular]] life forms, the bacteria of the genus ''[[Mycoplasma]]'', are around 200&nbsp;nm in length. By convention, nanotechnology is taken as the scale range {{nowrap|1 to 100 nm}}, following the definition used by the American [[National Nanotechnology Initiative]]. The lower limit is set by the size of atoms (hydrogen has the smallest atoms, which have an approximately ,25&nbsp;nm [[kinetic diameter]]). The upper limit is more or less arbitrary, but is around the size below which phenomena not observed in larger structures start to become apparent and can be made use of.<ref>{{cite book| vauthors = Allhoff F, Lin P, Moore D |title=What is nanotechnology and why does it matter?: from science to ethics|pages=3–5|publisher=Wiley |year=2010|isbn=978-1-4051-7545-6 |oclc=830161740}}</ref> These phenomena make nanotechnology distinct from devices that are merely miniaturized versions of an equivalent [[macroscopic scale|macroscopic]] device; such devices are on a larger scale and come under the description of [[microtechnology]].<ref>{{cite book| vauthors = Prasad SK |title=Modern Concepts in Nanotechnology|pages=31–32|publisher=Discovery Publishing House|year=2008|isbn=978-81-8356-296-6 |oclc=277278905}}</ref>

To put that scale in another context, the comparative size of a nanometer to a meter is the same as that of a marble to the size of the earth.<ref name="NationalG">{{cite journal| vauthors = Kahn J | title=Nanotechnology|journal=National Geographic|volume=2006|issue=June|pages=98–119|year=2006}}</ref>

Two main approaches are used in nanotechnology. In the "bottom-up" approach, materials and devices are built from molecular components which [[self-assembly|assemble themselves]] chemically by principles of [[molecular recognition]].<ref name="ReferenceA">{{cite journal | vauthors = Kralj S, Makovec D | title = Magnetic Assembly of Superparamagnetic Iron Oxide Nanoparticle Clusters into Nanochains and Nanobundles | journal = ACS Nano | volume = 9 | issue = 10 | pages = 9700–7 | date = October 2015 | pmid = 26394039 | doi = 10.1021/acsnano.5b02328 }}</ref> In the "top-down" approach, nano-objects are constructed from larger entities without atomic-level control.<ref>{{cite journal|journal=Nature Nanotechnology| vauthors = Rodgers P |year=2006|title=Nanoelectronics: Single file|doi=10.1038/nnano.2006.5|doi-access=free}}</ref>

Areas of physics such as [[nanoelectronics]], [[nanomechanics]], [[nanophotonics]] and [[nanoionics]] have evolved to provide nanotechnology's scientific foundation.

===Larger to smaller: a materials perspective===
[[File:Atomic resolution Au100.JPG|right|thumb|Image of [[Surface reconstruction|reconstruction]] on a clean [[Gold]]([[Miller index|100]]) surface, as visualized using [[scanning tunneling microscopy]]. The positions of the individual atoms composing the surface are visible.]]

{{main|Nanomaterials}}

Several phenomena become pronounced as system size. These include [[statistical mechanics|statistical mechanical]] effects, as well as [[Quantum mechanics|quantum mechanical]] effects, for example, the "[[quantum]] size effect" in which the electronic properties of solids alter along with reductions in particle size. Such effects do not apply at macro or micro dimensions. However, quantum effects can become significant when nanometer scales. Additionally, physical (mechanical, electrical, optical, etc.) properties change versus macroscopic systems. One example is the increase in surface area to volume ratio altering mechanical, thermal, and catalytic properties of materials. [[Diffusion]] and reactions can be different as well. Systems with fast ion transport are referred to as nanoionics. The mechanical properties of nanosystems are of interest in research.

===Simple to complex: a molecular perspective===
{{Main|Molecular self-assembly}}

Modern [[chemical synthesis|synthetic chemistry]] can prepare small molecules of almost any structure. These methods are used to manufacture a wide variety of useful chemicals such as [[drug|pharmaceuticals]] or commercial [[polymer]]s. This ability raises the question of extending this kind of control to the next-larger level, seeking methods to assemble single molecules into [[supramolecular assembly|supramolecular assemblies]] consisting of many molecules arranged in a well-defined manner.

These approaches utilize the concepts of molecular [[self-assembly]] and/or [[supramolecular chemistry]] to automatically arrange themselves into a useful conformation through a [[Top-down and bottom-up#Nanotechnology|bottom-up]] approach. The concept of [[molecular recognition]] is important: molecules can be designed so that a specific configuration or arrangement is favored due to [[Noncovalent bonding|non-covalent]] [[intermolecular force]]s. The Watson–Crick [[base pair|basepairing]] rules are a direct result of this, as is the specificity of an [[enzyme]] targeting a single [[substrate (biochemistry)|substrate]], or the specific [[protein folding|folding of a protein]]. Thus, components can be designed to be complementary and mutually attractive so that they make a more complex and useful whole.

Such bottom-up approaches should be capable of producing devices in parallel and be much cheaper than top-down methods, but could potentially be overwhelmed as the size and complexity of the desired assembly increases. Most useful structures require complex and thermodynamically unlikely arrangements of atoms. Nevertheless, many examples of self-assembly based on molecular recognition in exist in [[biology]], most notably Watson–Crick basepairing and enzyme-substrate interactions.

===Molecular nanotechnology: a long-term view===
{{Main|Molecular nanotechnology}}

Molecular nanotechnology, sometimes called molecular manufacturing, concerns engineered nanosystems (nanoscale machines) operating on the molecular scale. Molecular nanotechnology is especially associated with [[molecular assembler]]s, machines that can produce a desired structure or device atom-by-atom using the principles of [[mechanosynthesis]]. Manufacturing in the context of [[productive nanosystems]] is not related to conventional technologies used to manufacture nanomaterials such as carbon nanotubes and nanoparticles.

When Drexler independently coined and popularized the term "nanotechnology", he envisioned manufacturing technology based on [[molecular machine]] systems. The premise was that molecular-scale biological analogies of traditional machine components demonstrated molecular machines were possible: biology was full of examples of sophisticated, [[stochastic]]ally optimized [[Molecular machine#Biological|biological machines]].

Drexler and other researchers<ref>{{cite web| vauthors = Phoenix C |date=March 2005|url=http://www.crnano.org/developing.htm|title=Nanotechnology: Developing Molecular Manufacturing|archive-url=https://web.archive.org/web/20200601095107/http://www.crnano.org/developing.htm|archive-date=2020-06-01}}. crnano.org</ref> have proposed that advanced nanotechnology ultimately could be based on mechanical engineering principles, namely, a manufacturing technology based on the mechanical functionality of these components (such as gears, bearings, motors, and structural members) that would enable programmable, positional assembly to atomic specification.<ref>{{cite web|url=http://www.imm.org/PNAS.html|title=Some papers by K. Eric Drexler|work=imm.org|url-status=live|archive-url=https://web.archive.org/web/20060411075149/http://www.imm.org/PNAS.html|archive-date=2006-04-11}}</ref> The physics and engineering performance of exemplar designs were analyzed in Drexler's book ''Nanosystems: Molecular Machinery, Manufacturing, and Computation''.<ref name=Nanotsystems />

In general, assembling devices on the atomic scale requires positioning atoms on other atoms of comparable size and stickiness. [[Carlo Montemagno]]'s view is that future nanosystems will be hybrids of silicon technology and biological molecular machines.<ref>{{cite web |title=Carlo Montemagno, Ph.D. |url=http://www.cnsi.ucla.edu/institution/personnel?personnel%5fid=105488 |archive-url=https://web.archive.org/web/20141008065938/http://faculty.cnsi.ucla.edu/institution/personnel?personnel%5fid=105488 |archive-date=2014-10-08 |website=California NanoSystems Institute (CNSI), University of California, Los Angeles (UCLA)}}</ref> [[Richard Smalley]] argued that mechanosynthesis was impossible due to difficulties in mechanically manipulating individual molecules.<ref>{{Cite journal |last=Smalley |first=Richard E. |date=2001 |title=Of Chemistry, Love and Nanobots |url=https://www.jstor.org/stable/26059339 |journal=Scientific American |volume=285 |issue=3 |pages=76–77 |doi=10.1038/scientificamerican0901-76 |jstor=26059339 |pmid=11524973 |bibcode=2001SciAm.285c..76S |issn=0036-8733}}</ref>

This led to an exchange of letters in the [[American Chemical Society|ACS]] publication [[Chemical & Engineering News]] in 2003.<ref>{{cite journal | vauthors = Baum R |url=http://pubs.acs.org/cen/coverstory/8148/8148counterpoint.html|title=Cover Story – Nanotechnology|date=December 1, 2003|volume=81|issue=48|journal=Chemical and Engineering News|pages=37–42}}</ref> Though biology clearly demonstrates that molecular machines are possible, non-biological molecular machines remained in their infancy. [[Alex Zettl]] and colleagues at Lawrence Berkeley Laboratories and UC Berkeley<ref>{{cite web|url=http://research.physics.berkeley.edu/zettl/|archive-url=https://web.archive.org/web/20151008062820/http://research.physics.berkeley.edu/zettl/|archive-date=2015-10-08|title=Zettl Research Group |publisher=Department of Physics, University of California, Berkeley}}</ref> constructed at least three molecular devices whose motion is controlled via changing voltage: a nanotube [[nanomotor]], a molecular actuator,<ref>{{cite journal | vauthors = Regan BC, Aloni S, Jensen K, Ritchie RO, Zettl A | title = Nanocrystal-powered nanomotor | journal = Nano Letters | volume = 5 | issue = 9 | pages = 1730–3 | date = September 2005 | pmid = 16159214 | doi = 10.1021/nl0510659 | url = http://www.physics.berkeley.edu/research/zettl/pdf/312.NanoLett5regan.pdf | url-status = dead | osti = 1017464 | bibcode = 2005NanoL...5.1730R | archive-url = https://web.archive.org/web/20060510143208/http://www.physics.berkeley.edu/research/zettl/pdf/312.NanoLett5regan.pdf | archive-date = 2006-05-10 }}</ref> and a nanoelectromechanical relaxation oscillator.<ref>{{cite journal|url=http://www.lbl.gov/Science-Articles/Archive/sabl/2005/May/Tiniest-Motor.pdf|doi=10.1063/1.1887827|title=Surface-tension-driven nanoelectromechanical relaxation oscillator|year=2005| vauthors = Regan BC, Aloni S, Jensen K, Zettl A |journal=Applied Physics Letters |volume=86 |page=123119 |bibcode=2005ApPhL..86l3119R|issue=12|url-status=live|archive-url=https://web.archive.org/web/20060526193318/http://www.lbl.gov/Science-Articles/Archive/sabl/2005/May/Tiniest-Motor.pdf|archive-date=2006-05-26}}</ref>

Ho and Lee at [[Cornell University]] in 1999 used a scanning tunneling microscope to move an individual carbon monoxide molecule (CO) to an individual iron atom (Fe) sitting on a flat silver crystal and chemically bound the CO to the Fe by applying a voltage.<ref>{{Cite journal |last1=Lee |first1=H. J. |last2=Ho |first2=W. |date=1999-11-26 |title=Single-Bond Formation and Characterization with a Scanning Tunneling Microscope |url=https://www.science.org/doi/10.1126/science.286.5445.1719 |journal=Science |language=en |volume=286 |issue=5445 |pages=1719–1722 |doi=10.1126/science.286.5445.1719 |pmid=10576735 |issn=0036-8075}}</ref>