The mechanically controlled break junction (MCBJ) technique is an approach to contact (electrically) individual molecules. The idea is to fabricate nanocontacts by pulling a ductile metal wire until it narrows down and eventually breaks. Nanocontacts are usually fabricated on the flexible substrate, and the pulling is performed by its controlled bending. The key parameter of such system is an reduction, or attenuation, factor ($a = \Delta d/\Delta z$) between relative electrode displacement and bending of the substrate. The attenuation factor is defined by the geometry of the sample, and roughly can be estimated using the formula: $a = {6uT}/{L^2}$, where u is a  length of free-standing metallic bridge, T is a thickness of the substrate, and L is a  distance between side counter supports.

Historically, the first MCBJ devices were fabricated using notched metallic wire fixed on the substrate with epoxy.  Such approach allows very easy fabricate the device without any special equipment, which allows to observe quantized conductance in metallic wires. However, the attenuation factor of notched-wire MCBJ is very small and reaches only about $10^{-2}$.

Usage of lithographic techniques to fabricate MCBJ became very important improvement in the field. Towards that end, we prepattern metallic contacts via electron beam lithography on a flexible substrate (typically steel covered with polyimide) which can be placed into a three-point bending mechanism.
For our geometry, this leads to the attenuation factor on the order of  $10^{-4}$. This allows us to tune the elongation of the wire and the resulting gap after it breaks with sub-angström precision.

Finally, molecules can be added to the system via attaching a liquid cell with molecules in solution. When the wire breaks open, one or more molecules can spontaneously assemble between the contacts to form a molecular junction, which then can be studied using electronic and optoelectronic techniques.

Relevant papers:

• Ordered nanoparticles arrays interconnected by molecular wires: electronic and optoelectronic properties
Liao, J., S. Blok, S. J. van der Molen, S. Diefenbach, A. Holleitner, C. Schönenberger, A. Vladyka, and M. Calame.
Chem. Soc. Rev., 44(4), 999-1014 (2015).
[Abstract]

Arrays of metal nanoparticles in an organic matrix have attracted a lot of interest due to their diverse electronic and optoelectronic properties. Recent work demonstrates that nanoparticle arrays can be utilized as a template structure to incorporate single molecules. In this arrangement{,} the nanoparticles act as electronic contacts to the molecules. By varying parameters such as the nanoparticle material{,} the matrix material{,} the nanoparticle size{,} and the interparticle distance{,} the electronic behavior of the nanoparticle arrays can be substantially tuned and controlled. Furthermore{,} via the excitation of surface plasmon polaritons{,} the nanoparticles can be optically excited and electronically read-out. The versatility and possible applications of well-ordered nanoparticle arrays has been demonstrated by the realization of switching devices triggered optically or chemically and by the demonstration of chemical and mechanical sensing. Interestingly{,} hexagonal nanoparticle arrays may also become a useful platform to study the physics of collective plasmon resonances that can be described as Dirac-like bosonic excitations.

• Random telegraph signals in molecular junctions
Brunner, J., Teresa M. González, C. Schönenberger, and M. Calame.
Journal of Physics: Cond. Matt., 26, 474202 (2014).
• Regulating a Benzodifuran Single Molecule Field Effect Transistor via Electrochemical Gating and Optimization of Molecule/Electrode Coupling
Li, Zhihai, Hui Li, Songjie Chen, Toni Froehlich, Christian Schoenenberger, Michel Calame, Silvio Decurtins, Shi-Xia Liu, and Eric Borguet.
J. Am. Chem. Soc., 136 (25), 8867-8870 (2014).
• High-yield fabrication of nm-size gaps in monolayer CVD graphene
Nef, Cornelia, Laszlo Posa, Peter Makk, Wangyang Fu, Andras Halbritter, Christian Schönenberger, and Michel Calame.
Nanoscale, 6, 7249-7254 (2014).
• Feature in "Visions for a molecular future" (Brief viewpoint on molecular electronics)
Calame, Michel.
Nature Nanotechnology, 8, 385-389 (2013).
• Force-conductance correlation in individual molecular junctions
Nef, C., P. L. T. M. Frederix, Jan Brunner, C. Schönenberger, and M. Calame.
Nanotechnology, 23, 365201 (2012).
[Abstract]

Conducting atomic force microscopy is an attractive approach enabling the correlation of mechanical and electrical properties in individual molecular junctions. Here we report on measurements of gold–gold and gold–octanedithiol–gold junctions. We introduce two-dimensional histograms in the form of scatter plots to better analyze the correlation between force and conductance. In this representation, the junction-forming octanedithiol compounds lead to a very clear step in the force–conductance data, which is not observed for control monothiol compounds. The conductance found for octanedithiols is in agreement with the idea that junction conductance is dominated by a single molecule.

• Novel Cruciform Structures as Model Compounds for Coordination Induced Single Molecule Switches
Grunder, Sergio, Roman Huber, Songmei Wu, Christian Schönenberger, Michel Calame, and Marcel Mayor.
CHIMIA Int. J. Chem, 64, 140-144 (2010).
• Cyclic conductance switching in networks of redox-active molecular junctions
Liao, J., J. S. Agustsson, S. Wu, O. Jeannin, Y. -F. Ran, S. -X. Liu, S. and Decurtins, Y. Leroux, and M. Mayor<em>, et al</em>.
Nano Letters, 10 (3), 759-764 (2010).
• Oligoaryl Cruciform Structures as Model Compounds for Coordination-Induced Single-Molecule Switches
Grunder, Sergio, Roman Huber, Songmei Wu, Christian Schönenberger, Michel Calame, and Marcel Mayor.
Eur. J. Org. Chem., 5, 833-845 (2010).
• Molecular junctions: from tunneling to function
Calame, Michel.
CHIMIA Int. J. Chem, 64 (6), 391-397 (2010).
• Molecular Junctions based on Aromatic Coupling
Wu, S., M. -T. Gonzalez, R. Huber, S. Grunder, M. Mayor, Ch. Schoenenberger, and M. Calame.
Swiss Physical Society Communications, 26, 10 (2009).
[Abstract]

If individual molecules are to be used as building blocks for electronic devices, it will be essential to understand charge transport at the level of single molecules. Most existing experiments rely on the synthesis of functional rod-like molecules with chemical linker groups at both ends to provide strong, covalent anchoring to the source and drain contacts. This approach has proved very successful, providing quantitative measures of single-molecule conductance, and demonstrating rectification and switching at the single-molecule level. However, the influence of intermolecular interactions on the formation and operation of molecular junctions has been overlooked. Here we report the use of oligophenylene ethynylene molecules as a model system, and establish that molecular junctions can still form when one of the chemical linker groups is displaced or even fully removed. Our results demonstrate that aromatic pi-coupling between adjacent molecules is efficient enough to allow for the controlled formation of molecular bridges between nearby electrodes.

• Light-controlled conductance switching of ordered metal-molecule-metal devices
van der Molen, S. J., J. Liao, T. Kudernac, J. S. Agustsson, L. Bernard, M. Calame, B. J. van Wees, B. Ferringa, and C. Schönenberger.
Nano Letters, 9, 76-80 (2009).
• Molecular Junctions based on aromatic coupling
Wu, Songmei, Roman Huber, Teresa M. Gonzalez, Sergio Grunder, Marcel Mayor, Christian Schönenberger, and Michel Calame.
Nature Nanotechnology, 3(9), 569-574 (2008).
• Interlinking Au nanoparticles in 2D arrays via conjugated dithiolated molecules
Liao, Jianhui, Markus Mangold, Sergio Grunder, Marcel Mayor, Christian Schönenberger, and Michel Calame.
New J. Phys., 10, 65019 (2008).
• Conductance values of alkanedithiol molecular junctions
González, Teresa M., Jan Brunner, Roman Huber, Songmei Wu, Christian Schönenberger, and Michel Calame.
New J. Phys., 10, 65018 (2008).
• Electrical Conductance of Conjugated Oligomers at the Single Molecule Level
Huber, R., M. T. Gonzalez, S. Wu, M. Langer, S. Grunder, V. Horhoiu, M. Mayor, M. R. Bryce, and C. Wang<em>, et al</em>.
J. Am. Chem. Soc., 130(3), 1080-1084 (2008).
• Scaling of 1/f noise in tunable break junctions
Wu, Z. M., S. Wu, S. Oberholzer, M. Steinacher, M. Calame, and C. Schönenberger.
Phys. Rev. B, 78, 235421 (2008).
• Spectroscopy of Molecular Junction Networks Obtained by Place Exchange in 2D Nanoparticle Arrays
Bernard, L., Y. Kamdzhilov, M. Calame, S. J. van der Molen, J. Liao, and C. Schönenberger.
J. Phys. Chem. C, 111(50), 18445-18450 (2007).
• Tetrathiafulvalene-based moleculer electrical wires
Giacalone, Francesco, Angeles M. Herranz, Lucia Grueter, Teresa M. Gonzalez, Michel Calame, Christian Schönenberger, Carlos R. Arroyo, Gabino Rubio-Bollinger, and Marisela Velez<em>, et al</em>.
Chem. Comm., 46, 4854-4856 (2007).
• New Cruciform Structures: Toward Coordination Induced Single Molecule Switches
Grunder, S., R. Huber, V. Horhoiu, M. T. Gonzalez, C. Schönenberger, M. Calame, and M. Mayor.
J. Org. Chem., 72(22), 8337-8344 (2007).
• Schaltende Moleküle
Schoenenberger, Christian, Michel Calame, and Marcel Mayor.
UniNova, Wissenschaftsmagazin der Universität Basel, 103, 22-24 (2006).
• Building break junctions for molecular electronics
Schoenenberger, Christian and Michel Calame.
NanoNews, Newsletter of the NCCR Nanoscale Science, 01, 26-27 (2006).
• Electrical conductance of molecular junctions by a robust statistical analysis
González, Teresa M., Songmei Wu, Roman Huber, Sense Jan van der Molen, Christian Schönenberger, and Michel Calame.
Nano Lett., 6(10), 2238-2242 (2006).
• Reversible formation of molecular junctions in 2D nanoparticles arrays
Liao, Jianhui, Laetitia Bernard, Michael Langer, Christian Schönenberger, and Michel Calame.
Adv. Mat., 18 (18), 2444-2447 (2006).
• Resonant tunnelling through a C60 molecular junction in a liquid environment
Grueter, Lucia, Fuyong Cheng, Tero T. Heikkilä, M. Teresa Gonzalez, François Diederich, Christian Schönenberger, and Michel Calame.
Nanotech., 16, 2143 (2005).
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