graphene

In a new study, published in Science May 31, 2013, Columbia Engineering researchers demonstrate that graphene, even if stitched together from many small crystalline grains, is almost as strong as graphene in its perfect crystalline form. This work resolves a contradiction between theoretical simulations, which predicted that grain boundaries can be strong, and earlier experiments, which indicated that they were much weaker than the perfect lattice.

Graphene consists of a single atomic layer of carbon, arranged in a honeycomb lattice. “Our first Science paper, in 2008, studied the strength graphene can achieve if it has no defects—its intrinsic strength,” says James Hone, professor of mechanical engineering, who led the study with Jeffrey Kysar, professor of mechanical engineering. “But defect-free, pristine graphene exists only in very small areas. Large-area sheets required for applications must contain many small grains connected at grain boundaries, and it was unclear how strong those grain boundaries were. This, our second Science paper, reports on the strength of large-area graphene films grown using chemical vapor deposition (CVD), and we’re excited to say that graphene is back and stronger than ever.”

The study verifies that commonly used methods for post-processing CVD-grown graphene weaken grain boundaries, resulting in the extremely low strength seen in previous studies. The Columbia Engineering team developed a new process that prevents any damage of graphene during transfer. “We substituted a different etchant and were able to create test samples without harming the graphene,” notes the paper’s lead author, Gwan-Hyoung Lee, a postdoctoral fellow in the Hone lab. “Our findings clearly correct the mistaken consensus that grain boundaries of graphene are weak. This is great news because graphene offers such a plethora of opportunities both for fundamental scientific research and industrial applications.”

James Hone

Jeffrey Kysar

In its perfect crystalline form, graphene (a one-atom-thick carbon layer) is the strongest material ever measured, as the Columbia Engineering team reported in Science in 2008—so strong that, as Hone observed, “it would take an elephant, balanced on a pencil, to break through a sheet of graphene the thickness of Saran Wrap.” For the first study, the team obtained small, structurally perfect flakes of graphene by mechanical exfoliation, or mechanical peeling, from a crystal of graphite. But exfoliation is a time-consuming process that will never be practical for any of the many potential applications of graphene that require industrial mass production.

Currently, scientists can grow sheets of graphene as large as a television screen by using chemical vapor deposition (CVD), in which single layers of graphene are grown on copper substrates in a high-temperature furnace. One of the first applications of graphene may be as a conducting layer in flexible displays.

“But CVD graphene is ‘stitched’ together from many small crystalline grains—like a quilt—at grain boundaries that contain defects in the atomic structure,” Kysar explains. “These grain boundaries can severely limit the strength of large-area graphene if they break much more easily than the perfect crystal lattice, and so there has been intense interest in understanding how strong they can be.”

The Columbia Engineering team wanted to discover what was making CVD graphene so weak. In studying the processing techniques used to create their samples for testing, they found that the chemical most commonly used to remove the copper substrate also causes damage to the graphene, severely degrading its strength.

Their experiments demonstrated that CVD graphene with large grains is exactly as strong as exfoliated graphene, showing that its crystal lattice is just as perfect. And, more surprisingly, their experiments also showed that CVD graphene with small grains, even when tested right at a grain boundary, is about 90% as strong as the ideal crystal.

“This is an exciting result for the future of graphene, because it provides experimental evidence that the exceptional strength it possesses at the atomic scale can persist all the way up to samples inches or more in size,” says Hone. “This strength will be invaluable as scientists continue to develop new flexible electronics and ultrastrong composite materials.”

Strong, large-area graphene can be used for a wide variety of applications such as flexible electronics and strengthening components—potentially, a television screen that rolls up like a poster or ultrastrong composites that could replace carbon fiber. Or, the researchers speculate, a science fiction idea of a space elevator that could connect an orbiting satellite to Earth by a long cord that might consist of sheets of CVD graphene, since graphene (and its cousin material, carbon nanotubes) is the only material with the high strength-to-weight ratio required for this kind of hypothetical application.

The team is also excited about studying 2D materials like graphene. “Very little is known about the effects of grain boundaries in 2D materials,” Kysar adds. “Our work shows that grain boundaries in 2D materials can be much more sensitive to processing than in 3D materials. This is due to all the atoms in graphene being surface atoms, so surface damage that would normally not degrade the strength of 3D materials can completely destroy the strength of 2D materials. However with appropriate processing that avoids surface damage, grain boundaries in 2D materials, especially graphene, can be nearly as strong as the perfect, defect-free structure.”

The study was supported by grants from the Air Force Office of Scientific Research and the National Science Foundation 

Carbocation

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A carbocation is a molecule in which a carbon atom has a positive charge and three bonds. We can basically say that they are carbon cations. Formerly, it was known as carbonium ion. Carbocation today is defined as any even-electron cation that possesses a significant positive charge on the carbon atom.

Talking about some general characteristics, the carbon cations are very reactive and unstable due to an incomplete octet. In simple words, carbocations do not have eight electrons, therefore they do not satisfy the octet rule.

In carbocation, the hybridization of carbon will be sp² and its shape is trigonal planar. There is also a vacant p orbital which indicates its electron-deficient nature. The carbon has 6 electrons in its valence shell. Due to this, it is an electron-deficient species, also known as an electrophile.

A carbocation is generally observed in an SN1 reaction, elimination reaction, etc.

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Classification of Carbocation

The different carbocations are named on the basis of the number of carbon groups bonded to the carbon. The carbocation can be termed as methyl, primary, secondary or tertiary on the basis of how many carbon atoms are attached to it:

• Methyl carbocation: If no carbon is attached to the carbon with the positive charge it is simply called as methyl carbocation.
• If one, two or three carbon is attached to the carbon with the positive charge it is called the primary carbocation, secondary carbocation, and tertiary carbocation respectively.

• If there is a carbon-carbon double bond next to the carbon with the positive charge it is termed as allylic carbocation.

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• In the same way, if the carbon with the positive charge is attached to a double bond, the carbocation is termed as vinylic carbocation. Here, the hybridization of the carbon having the positive charge is sp and geometry is linear.
• Whenever the carbon which consists of the positive charge is part of a benzene ring, then the carbocation an aryl carbocation.
• If the carbon having a positive charge is immediately next to a benzene ring, it is termed as a benzylic carbocation.

Interestingly, in addition to these types, there is another type of carbocation which is known as pyramidal carbocation. In this type, the ions consist of a single carbon atom that usually tends to hover over a four- or five-sided polygon which can be depicted as a pyramid. The 4 sided pyramidal ion will consist of +1 charge while the five-sided pyramid will have +2 charge.

Formation of the Carbocation

The carbocations can be formed by either of the following two fundamental steps:

• Cleavage of a bond of carbon.
• Electrophilic addition.

Cleavage of Bond of Carbon

Whenever there is a cleavage of the bond of carbon and atoms attached to it, the leaving group takes away the shared electrons. Thus leaving the carbon atom as electron deficient. As a result, a positive charge is developed forming a carbocation. The more tendency of cleavage of bond or formation of a more stable carbocation the lower is the activation energy.

In many organic reactions such as the SN1 and E1 reactions, carbocation is formed as a reaction intermediate.

Electrophilic Addition

In electrophilic addition, an electrophile attacks on an unsaturated point(double or triple bond), which results in the breaking of the pi bond which results in the formation of a carbocation. The more stable is the carbocations the lower is the activation energy and faster the addition. Electrophilic addition to a pi bond is illustrated by the reaction of HBr (an electrophile) with propene (CH₃CH = CH₂).

It can be noted that the formation of the secondary carbocation is favoured over the primary carbocation because secondary carbocation is more stabilized due to resonance. This is also in accordance with Markovnikov’s Rule. Such electrophilic addition reactions are generally seen in alkenes, alkynes and benzene rings.

As we know that the carbocations are very reactive due to their electron deficiency, vacant orbital and incomplete octet. Therefore, its stability depends on the octet completion and reducing the electron deficiency.

The stability of a carbocation can be achieved by the following processes:

(a) Addition of a nucleophile.

(b) Formation of a pi bond.

(c) Rearrangement.

Addition of a Nucleophile

A carbocation is electron-deficient and with an incomplete octet and a positive charge on it. The positive charge is stabilized by the addition of a nucleophile thus the formation of a new covalent bond takes place. This stabilizes the carbocation. This is a very common process of stabilization of carbocation because the carbocation is very reactive so even weak nucleophile gets attached to the carbocation.

Formation of a Pi Bond

The carbocation can receive electrons from nearby hydrogen to remove its positive charge and to complete its octet. Thus a new pi bond can be formed. The hydrogen atom is generally must be removed by any base. Due to the high reactivity of the carbocations even a weak base such as water or iodide ion are able to facilitate the deprotonation. Whenever such deprotonation occurs two types of products are formed. The more stable compound is the major product.

Rearrangement

The bonding electrons of a carbocation can be shifted between adjacent atoms so that a more stable carbocation can be formed. For instance, rearrangement will be highly favoured if there is a conversion of a secondary carbocation that can be formed from a primary carbocation The reason is simple because the carbocation is more stabilized in secondary carbocation than in a primary carbocation.

The different types of carbocation rearrangement are:

Hydride Shifting

Here hydrogen is shifted from 1st carbon to 2nd carbon. So the carbocation has changed from primary to the secondary carbocation. Thus forming a more stable structure.

Methyl shifting

Here methyl group shifts to the primary carbon to form a more stable structure. The carbocation is secondary carbocation, so more stable than primary carbocation.

Phenyl shifting

The entire phenyl group can also be shifted to obtain a more stable secondary or tertiary carbocation than a primary carbocation. This is also interesting to know that a phenyl shift is more favoured than a methyl shift.

Carbocation Stability

The stability order of carbocation is as follows:

The stability of carbocations depends on the following factors:

1. Resonance: Stability of carbocations increases with the increasing number of resonances. More the number of resonating structures more is the stability of the carbocation. The reason for this is the delocalization of the positive charge. The electron deficiency is decreased due to the delocalization and thus it increases the stability.

When compared to substitution, the resonance effect proves to be a more dominating factor than substitution. Therefore, structures with resonance are far better stabilised than others. Cyclopropane carbocation is exceptionally very stable due to dancing resonance. Thus tricyclo propane carbocation is the most stable carbocation.

2. Hyperconjugation and inductive effect: Increasing substitution, increases the hyperconjugation and thus it increases stability. More the hyperconjugation more is the stability.

R₃C⁺ (3^o ; most stable) > R₂CH⁺ (2^o ) > RCH₂⁺ (1^o) CH₃⁺ (methyl; least stable)

The Carbocation stability depends on the number of carbon groups attached to the carbon carrying the positive charge.

3. Electronegativity: Electronegativity indicates the capacity of an atom to attract electrons. The more is the electronegativity, the more is the attraction of the electrons towards the atom. Therefore the electronegativity of the carbon with the positive directly affects the stability of the carbocation. So as the electronegativity of the carbon atom increases the stability of the carbocation decreases. sp > sp² > sp³ ( sp has maximum s character; so maximum electronegativity, sp³ has minimum s character; so minimum electronegativity).

The hybridisation of the carbon with the positive charge in the vinylic carbocation is sp whose electronegativity is more than the sp² hybridized carbon of the alkyl carbocation. Due to this reason, the stability of a primary vinylic carbocation is less than a primary alkyl carbocation.

In the same way, lower stability of aryl carbocation in comparison to a secondary alkyl carbocation can be explained. Vinyl and aryl carbocations are very rare to find due to their low stability.

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फिर मुझे तू मिल गया

फिर मुझे तू मिल गया

ना मेरे पास गाड़ी, बंगला आलीशान

खाली जेब, पैसों का न नामोनिशान।

ना कोई हंसता मेरी बहकी बातों पे

ना कोई संग मेरी अकेली रातों में।

दिल में सोच के अपनाली थी सच्चाई हमने,

कि ना मिलेगा जो लुटाए हमपे प्यार।

दूजो को छोड़ो, खुद ना दिखती थी अच्छाई हममें,

तो मुमकिन हो कैसे दिलों का कारोबार।

पर फिर मुझे तू मिल गया, पर फिर मुझे तू मिल गया

मुझे थी जिसकी जुस्तजू मिल गया, चाहा था जैसा वहीं मिल गया।

ज्यादा सयाना ना भोलेपन का हैं रोग, ना सूरत ऐसी कि पलट के देखें लोग।

मिल जाऊं भीड़ में आसानी से, बस यूंही कट रहीं थी जिंदगानी ये।।

मिलना नामुमकिन था इस मतलबी जमाने में वो,

जो करदे कमी खामियों को दरकिनार।

नाता पुराना सा जिस अजनबी अनजाने से हो,

जो मेरी जिंदगी इश्क से दे सवार।।

पर फिर मुझे तू मिल गया, पर फिर मुझे तू मिल गया

मुझे थी जिसकी जुस्तजू मिल गया, चाहा था जैसा वहीं मिल गया।

जल्दी मिलाएगा रब उससे रास्ते, जिसे बनाया हो बस तेरे वास्ते।

जो दे तुम्हारी बेकरारी को करार, बस करना होगा इस घड़ी का इंतज़ार।

यकीं इन सब बातो से उठ चुका था इस कदर कि

अनोखी लगती थी मोहब्बत की सौगात।

रेखा हाथों में ही नहीं थी किसी हमसफर की,

तो फिर कैसे हो जाती मुलाकात।

पर फिर मुझे तू मिल गया, पर फिर मुझे तू मिल गया

मुझे थी जिसकी जुस्तजू मिल गया, चाहा था जैसा वहीं मिल गया।